Can't see the blunder

First post, apologies if I don't follow standard form.

I have an old copy of Chessmaster and I've been trying to self-study using its tutorials and practice avatars and I ended up playing this game as black.

1. e3 d5 2. Nc3 e5 3. Qf3 Nc6 4. Qg3 Nf6 5. Bb5 Bd7 6. Bxc6 Bxc6 7. Qxe5+
Be7 8. Nge2 O-O 9. Qd4 b6 10. Ng3 Bc5 11. Qh4 Re8 12. b4 Be7 13. Nf5 d4 14.
Ne2 Bxb4 15. Nexd4 Be4 16. a3 c5 17. Qg5 Bxd2+ 18. Bxd2 Bxf5 19. Nxf5 Qxd2+
20. Kxd2 Ne4+ 21. Ke2 Nxg5 22. Rad1 Ne4 23. Rd7 Rad8 24. Rxd8 Rxd8 25. Kf3
Nd2+ 26. Ke2 Nc4 27. Rd1 Rxd1 28. Kxd1 Kf8 29. Nxg7 Kxg7 30. Kc1 Nxa3 31.
h4 Nc4 32. g4 Kf6 33. g5+ Kf5 34. Kd1 Kg4 35. Ke2 a5 36. Kd3 Ne5+ 37. Ke4

You can stop laughing now...

anyhow, I was under time pressure so I played ... Nc4 and both CM and Stockfish tell me its a serious blunder but I don't see it.  I'm sure it wasn't the best move but after 38 Kd3 b5 the a file pawn will Queen and white can't do anything about it as far as I can tell without losing the entire King side of the board.

Any insight would be appreciated.

Tom.

Computer programs are extremely materialistic, don't understand anything except tactics that lie within their search range, and are particularly weak in endgames. That should be explanation enough.

The engines probably want you to just push the a pawn instead of Nc4.... with the knight on e5 the king can't chase the pawn down.

Thanks.

Elementary endgame technique.  In the position displayed the White K is outside the Black a pawn's queening square.  And, it cannot get inside the  queening square.  Because the Black N prevents the White K from moving to d3, and the Black pawn at c5 prevents the White K from moving to d4.  The d3 and d4 squares are the only  squares  that would  allow the White K to  get inside the  queening square and  stop the  a pawn from queening.

In other words, after 37...a4 White cannot stop the Black a pawn from queening.  Also, there is no counterplay in the position whereby White could threaten to queen a pawn of his own.  Niether thru direct or indirect defense can White prevent the  Black a pawn from queening.

So it was just the act of letting the white king back in the box that was so terrible?  Weird.  I only discovered the "blunder" when I ran the game through stockfish after the game.  During the game I was just hoping to save the knight but a few variations I tried led to losing the knight and a pawn or two but you'd really have to bork it up to lose the a pawn...

As you can tell I have a lot to learn about "efficient" endgames...

Thanks again.