pgn-extract doesn't see any issue with it, but pasting into Tarrasch Chess GUI offers a clue that it may be the null move indicator:
16. exf4 Z0 (16... Qxd6 $1
Null moves are not part of the pgn standard, it's hard to complain about software that does things differently there, or even doesn't support null moves at all. I have seen both "Z0" and "--", you can try changing to the second one and see if that makes chess.com happy.
Null moves have always been a problem, but I removed those I found. On my last effort, I spotted the Z0 and removed it, still to no avail.
Did you spot there were three Z0's? I removed all three and chess.com takes the pgn:
[Event "USSR U-20 Championship, Riga"]
[Site "?"]
[Date "1977.??.??"]
[Round "?"]
[White "Kasparov, G."]
[Black "Kengis, E."]
[Result "1-0"]
[ECO "A48"]
[Annotator "Colin Crouch"]
[PlyCount "45"]
[EventDate "1977.??.??"]
[SourceTitle "CS: Great Attackers"]
[Source "Everyman Chess"]
[SourceDate "2011.11.01"]
[SourceVersion "1"]
[SourceVersionDate "2011.11.01"]
[SourceQuality "1"]
1. d4 Nf6 2. Nf3 b6 3. Bf4 Bb7 4. e3 c5 5. Nbd2 g6 6. c3 Bg7 7. h3 O-O 8. Be2
Nc6 9. O-O d6 10. a4 {It's so quiet... Nevertheless, with all the pieces and
pawns still on the board, there is no prospect of an immediate draw, unless
both players are happy with an early cessation of conflict.} a6 {In 'Fighting
Chess', Kasparov was critical of this move, regarding it as a slight weakening
of the pawn on b6, and the square itself.} ({Kasparov himself suggested} 10...
Na5 11. b4 cxb4 12. cxb4 Nc6 13. Qb3 e5 {as equal, while others have tried
more direct ways of playing for ...e7-e5, such as via 10...Re8 and ...Qc7.}) ({
Black was already better, for example, after} 10... Re8 11. Nc4 Qc7 12. Rc1
Rad8 13. dxc5 bxc5 14. Qc2 e5 {, in P.Pcola-L.Salai, Slovakian Championship,
Martin 1996, although no doubt White can play slightly more accurately. On the
other hand, there must be greater mistakes in the game for Black's position to
collapse by move 23.}) 11. Nc4 b5 {There are of course other moves, Fritz
giving a dozen or so equal suggestions.} ({For instance,} 11... d5 $5 12. Nce5
Ne4 {is quite direct, while Kengis also plays sharply.}) 12. Na3 b4 13. cxb4
Nxb4 {It's the one chance for Kengis to try to close the gap on the tournament
leader, and he would presumably be reasonably happy to keep up pressure on the
long dark-squared diagonal.} ({Otherwise} 13... cxb4 {is equal.}) ({Not,
though,} 13... cxd4 $2 14. b5 $1 {with advantage to White.}) 14. dxc5 $1 {
Kasparov decides to open up the game. This passes with comment in his own
notes, but it is in fact highly committal, for good or bad. White does not
have to open the long diagonal, and could simply block with one of many quiet
moves, but Black would then be comfortably equal.} Nfd5 (14... dxc5 $2 15. Rc1
Nfd5 16. Be5 {leaves Black with weaknesses, so he must sacrifice.}) 15. cxd6 {
Comparing the last two diagrams, it is amazing how quickly the pawn centre has
cleared. The position has not been stabilized yet, and so there are tactics
likely to come up.} Bxb2 $6 {Spare a thought for Kengis. He has worked hard,
played imaginatively, made pawn sacrifices as Black to open up the centre,
cleared both diagonals for his fianchettoed bishops, and found two strong
squares for the knights. Now he regains one of his pawns, planning to win the
exchange, and everything looks good. But everything crumbles after a hidden
resource.} ({Black's position is still playable at this point, but he needs to
cut down his ambitions. After} 15... Nxf4 16. exf4 16... Qxd6 $1
({.
Kasparov mentions the more ambitious} 16... Bxb2 {for Black, but as he notes,}
17. Qb3 Bxf3 18. Bxf3 Bxa1 19. dxe7 Qxe7 20. Bxa8 Bd4 {is somewhat better for
White. Offering the queen exchange with 16...Qxd6! is preferable.})
17. Qxd6
exd6 {, Black is a pawn down, with the queens exchanged, but he has a good
bishop pair. White's f-pawns are doubled, and cannot provide any massive edge
on the kingside, while his queenside pawns are vulnerable. This gives
indications that Black probably has excellent drawing chances. For example,
after the continuation} 18. Nc4 Rfe8 19. Nxd6 Rxe2 20. Nxb7 {, play is roughly
balanced. It is probably unwise for either player to try for more.}) 16. Bh6 $1
{He keeps the bishop.} ({Instead,} 16. dxe7 $6 Qxe7 17. Nc4 Nxf4 18. Nxb2 Rfd8
{favours Black.}) 16... Re8 $2 ({The younger Kasparov suggests, in 'Fighting
Chess', that White has a clear edge after} 16... exd6 17. Bxf8 Qxf8 18. Qd2
18... Qg7
({. Alas,
there is a slip in this, as Fritz points out. After} 18... Qg7 $2 {, White has}
19. Rab1 $1 Bxa3 20. Rb3 {, keeping the extra exchange with a winning position.
})
({. However, this can easily be amended with} 18... Bc3 $1 19. Qc1 Bxa1 20.
Qxa1 {, and no doubt Kasparov would have amended it himself had Fritz been
around, without much change in the substance. The more interesting question is
whether Black has any problems in holding the position in view of his isolated
d-pawn. If the queens stay on the board, Black is in serious trouble, as the
white queen is far more active, so he has to offer a trade with} Qg7 $1 {
. Play might continue} 21. Qxg7+ Kxg7 22. Rc1 Rc8 $1 {(both sides must take
care of the open c-file)} 23. Rxc8 Bxc8 {, and what saves Black here is that
the white a-pawn is itself isolated and weak, after for example} 24. Nc4 Bd7 $1
{.})
19. Nd4 Bc3 20. Qc1 Rc8 21. Nc4 Bxa1 22. Qxa1 Qf6 {.} )
17. d7 $3 {A later game in this collection, against Pribyl, involves
another unexpected pawn sacrifice, this time with d7-d8. Both moves are
striking, but have different motives. Kasparov's win against Kengis is
virtually inexplicable if one thinks of it in terms of an aggressive sacrifice.
Precisely because of this, his move has considerable charm. Let us look at
another point of view. White is a pawn up, but two of his pieces are under
attack by the bishop on b2. In addition, Black is threatening to attack the
queen with ...Nc3, with all three minor pieces buzzing around the white camp.
What then can White do? The answer is that he can return his extra pawn to
gain time. When Black captures on d7, his knight will be pinned against the
queen. This in itself gives a sufficient explanation why the pawn sacrifice is
at least worth thinking about. The aggressive knight is no longer effective.
White still has the problem of dealing with the threats from Black's bishop
and other knight, but this is not so great a problem. He can sacrifice the
exchange! After all, Black would then have no bishop on the long dark-squared
diagonal. So:} Qxd7 18. Nc4 $1 (18. Rb1 Bxa3 19. Rb3 Qxa4 20. Qa1 {could
perhaps be tried; for example,} e5 21. Rxa3 Qc2 $2 22. Nxe5 f6 23. Ng4 {
, but the exchange sacrifice is simpler, more direct, and more effective.})
18... Bxa1 19. Qxa1 e5 ({Do you remember how Kasparov criticized Black's
earlier 10...a6, weakening a queenside pawn? Suddenly this is of significance:
} 19... Nf6 20. Nb6 Qe6 21. Nxa8 {recovers the sacrificed exchange, and
White's bishop pair gives a clear edge when we go towards the endgame after}
Rxa8 22. Qe5 Qxe5 23. Nxe5 {.}) ({Also, if} 19... f6 {then} 20. e4 {causes
problems.}) 20. Ncxe5 {An important knight move. Now the diagonals take centre
stage.} Qe6 $6 {This leads to a quick finish.} ({If instead White tries the
line from the main game, then} 20... Qd6 21. Ng4 f6 22. Bc4 Kh8 $1 {creates a
problem, as after} 23. e4 $2 {, Black has simply}
(23. Ng5 Re7 24. e4 Nc2 $5 25. Qb2 Qb4 26.
Qxc2 Rc8 27. exd5 Rxc4 28. Qa2 fxg5 29. Bf8 {would be one way of finishing.})
23... Rxe4 {. Kasparov
would presumably find a way to win without too much difficulty, but at least
he would have to work a little.})
21. Ng4 f6 22. Bc4 Rf8 23. e4 {Pins wins!} 1-0
I often export a PGN from ChessBase to use in a lesson with my students. All too often I get this error when attempting to paste it in the analysis board.
Chess.com's recognition of what constitutes a valid PGN should be more robust.
Tell me what is wrong here:
[Event "USSR U-20 Championship, Riga"]
[Site "?"]
[Date "1977.??.??"]
[Round "?"]
[White "Kasparov, G."]
[Black "Kengis, E."]
[Result "1-0"]
[ECO "A48"]
[Annotator "Colin Crouch"]
[PlyCount "45"]
[EventDate "1977.??.??"]
[SourceTitle "CS: Great Attackers"]
[Source "Everyman Chess"]
[SourceDate "2011.11.01"]
[SourceVersion "1"]
[SourceVersionDate "2011.11.01"]
[SourceQuality "1"]
1. d4 Nf6 2. Nf3 b6 3. Bf4 Bb7 4. e3 c5 5. Nbd2 g6 6. c3 Bg7 7. h3 O-O 8. Be2
Nc6 9. O-O d6 10. a4 {It's so quiet... Nevertheless, with all the pieces and
pawns still on the board, there is no prospect of an immediate draw, unless
both players are happy with an early cessation of conflict.} a6 {In 'Fighting
Chess', Kasparov was critical of this move, regarding it as a slight weakening
of the pawn on b6, and the square itself.} ({Kasparov himself suggested} 10...
Na5 11. b4 cxb4 12. cxb4 Nc6 13. Qb3 e5 {as equal, while others have tried
more direct ways of playing for ...e7-e5, such as via 10...Re8 and ...Qc7.}) ({
Black was already better, for example, after} 10... Re8 11. Nc4 Qc7 12. Rc1
Rad8 13. dxc5 bxc5 14. Qc2 e5 {, in P.Pcola-L.Salai, Slovakian Championship,
Martin 1996, although no doubt White can play slightly more accurately. On the
other hand, there must be greater mistakes in the game for Black's position to
collapse by move 23.}) 11. Nc4 b5 {There are of course other moves, Fritz
giving a dozen or so equal suggestions.} ({For instance,} 11... d5 $5 12. Nce5
Ne4 {is quite direct, while Kengis also plays sharply.}) 12. Na3 b4 13. cxb4
Nxb4 {It's the one chance for Kengis to try to close the gap on the tournament
leader, and he would presumably be reasonably happy to keep up pressure on the
long dark-squared diagonal.} ({Otherwise} 13... cxb4 {is equal.}) ({Not,
though,} 13... cxd4 $2 14. b5 $1 {with advantage to White.}) 14. dxc5 $1 {
Kasparov decides to open up the game. This passes with comment in his own
notes, but it is in fact highly committal, for good or bad. White does not
have to open the long diagonal, and could simply block with one of many quiet
moves, but Black would then be comfortably equal.} Nfd5 (14... dxc5 $2 15. Rc1
Nfd5 16. Be5 {leaves Black with weaknesses, so he must sacrifice.}) 15. cxd6 {
Comparing the last two diagrams, it is amazing how quickly the pawn centre has
cleared. The position has not been stabilized yet, and so there are tactics
likely to come up.} Bxb2 $6 {Spare a thought for Kengis. He has worked hard,
played imaginatively, made pawn sacrifices as Black to open up the centre,
cleared both diagonals for his fianchettoed bishops, and found two strong
squares for the knights. Now he regains one of his pawns, planning to win the
exchange, and everything looks good. But everything crumbles after a hidden
resource.} ({Black's position is still playable at this point, but he needs to
cut down his ambitions. After} 15... Nxf4 16. exf4 Z0 (16... Qxd6 $1 17. Qxd6
exd6 {, Black is a pawn down, with the queens exchanged, but he has a good
bishop pair. White's f-pawns are doubled, and cannot provide any massive edge
on the kingside, while his queenside pawns are vulnerable. This gives
indications that Black probably has excellent drawing chances. For example,
after the continuation} 18. Nc4 Rfe8 19. Nxd6 Rxe2 20. Nxb7 {, play is roughly
balanced. It is probably unwise for either player to try for more.}) ({.
Kasparov mentions the more ambitious} 16... Bxb2 {for Black, but as he notes,}
17. Qb3 Bxf3 18. Bxf3 Bxa1 19. dxe7 Qxe7 20. Bxa8 Bd4 {is somewhat better for
White. Offering the queen exchange with 16...Qxd6! is preferable.})) 16. Bh6 $1
{He keeps the bishop.} ({Instead,} 16. dxe7 $6 Qxe7 17. Nc4 Nxf4 18. Nxb2 Rfd8
{favours Black.}) 16... Re8 $2 ({The younger Kasparov suggests, in 'Fighting
Chess', that White has a clear edge after} 16... exd6 17. Bxf8 Qxf8 18. Qd2 Z0
(18... Qg7 19. Nd4 Bc3 20. Qc1 Rc8 21. Nc4 Bxa1 22. Qxa1 Qf6 {.}) ({. Alas,
there is a slip in this, as Fritz points out. After} 18... Qg7 $2 {, White has}
19. Rab1 $1 Bxa3 20. Rb3 {, keeping the extra exchange with a winning position.
}) ({. However, this can easily be amended with} 18... Bc3 $1 19. Qc1 Bxa1 20.
Qxa1 {, and no doubt Kasparov would have amended it himself had Fritz been
around, without much change in the substance. The more interesting question is
whether Black has any problems in holding the position in view of his isolated
d-pawn. If the queens stay on the board, Black is in serious trouble, as the
white queen is far more active, so he has to offer a trade with} Qg7 $1 {
. Play might continue} 21. Qxg7+ Kxg7 22. Rc1 Rc8 $1 {(both sides must take
care of the open c-file)} 23. Rxc8 Bxc8 {, and what saves Black here is that
the white a-pawn is itself isolated and weak, after for example} 24. Nc4 Bd7 $1
{.})) 17. d7 $3 {A later game in this collection, against Pribyl, involves
another unexpected pawn sacrifice, this time with d7-d8. Both moves are
striking, but have different motives. Kasparov's win against Kengis is
virtually inexplicable if one thinks of it in terms of an aggressive sacrifice.
Precisely because of this, his move has considerable charm. Let us look at
another point of view. White is a pawn up, but two of his pieces are under
attack by the bishop on b2. In addition, Black is threatening to attack the
queen with ...Nc3, with all three minor pieces buzzing around the white camp.
What then can White do? The answer is that he can return his extra pawn to
gain time. When Black captures on d7, his knight will be pinned against the
queen. This in itself gives a sufficient explanation why the pawn sacrifice is
at least worth thinking about. The aggressive knight is no longer effective.
White still has the problem of dealing with the threats from Black's bishop
and other knight, but this is not so great a problem. He can sacrifice the
exchange! After all, Black would then have no bishop on the long dark-squared
diagonal. So:} Qxd7 18. Nc4 $1 (18. Rb1 Bxa3 19. Rb3 Qxa4 20. Qa1 {could
perhaps be tried; for example,} e5 21. Rxa3 Qc2 $2 22. Nxe5 f6 23. Ng4 {
, but the exchange sacrifice is simpler, more direct, and more effective.})
18... Bxa1 19. Qxa1 e5 ({Do you remember how Kasparov criticized Black's
earlier 10...a6, weakening a queenside pawn? Suddenly this is of significance:
} 19... Nf6 20. Nb6 Qe6 21. Nxa8 {recovers the sacrificed exchange, and
White's bishop pair gives a clear edge when we go towards the endgame after}
Rxa8 22. Qe5 Qxe5 23. Nxe5 {.}) ({Also, if} 19... f6 {then} 20. e4 {causes
problems.}) 20. Ncxe5 {An important knight move. Now the diagonals take centre
stage.} Qe6 $6 {This leads to a quick finish.} ({If instead White tries the
line from the main game, then} 20... Qd6 21. Ng4 f6 22. Bc4 Kh8 $1 {creates a
problem} 23. Z0 ({, as after} 23. e4 $2 {, Black has simply} Rxe4 {. Kasparov
would presumably find a way to win without too much difficulty, but at least
he would have to work a little.}) (23. Ng5 Re7 24. e4 Nc2 $5 25. Qb2 Qb4 26.
Qxc2 Rc8 27. exd5 Rxc4 28. Qa2 fxg5 29. Bf8 {would be one way of finishing.}))
21. Ng4 f6 22. Bc4 Rf8 23. e4 {Pins wins!} 1-0