Why my bishop move is a blunder ? I have a easy target on opponent's queen.

If I have captured opponent's knight, then he would have easily escaped from this situation.

I may be wrong, but I think it was marked as a blunder for a number of reasons:
1. The queen can easily move out of this so while it does attack the queen, you are already in the deep middle game so moving the queen to a powerful square like a5 in this situation means white does not lose a tempo and gains an advantage which lets them create many additional attacks, because of this, the move does not have any extremely damaging effects for white.

2. Secondly, the move cxd4 is good since that knight is a very active piece and capturing it will save you a lot of potential problems, as an example, if white managed to make you move your f7 pawn forward, they could play Ne6+ which forks the king and queen, since the f7 pawn moved you cannot capture the knight and you cannot protect using another piece since the check is with a knight, because of this you are forced to move your king away and will therefore lose your queen, this is probably the more important reason.

It looks as if your ...Bg5 pins the white queen to the king, winning! But White has a clever way out. He can play Nf5+ followed by Ne3 and your pin is blocked/ That's why taking the knight is better. Tricky!

No the queen can not move out of this because it's pinned, if it moves the king is in check, so it has to stay on that line, and even if it could move a5 is not good because blacks queen is on d8 covering that square so if Qa5 then black simply plays Qxa5.

 

The reason why it's a blunder is like they pointed out here that white can play nf5+ and then ne3 which stops the pin, so it's better to just take the free knight right away instead.

I love reading these forums, I never would have seen the Nf5 trick others pointed out.

+1

Neither me or by luck my opponent didnt know this one, so I was able to achieve what I planned. ☺️

But yes, now I underatand this is a blunder, the best move is just take the free knight.  

Thanks. 

Just Knight f 5 check, then knight e3, and the game continues with white up a pawn. Ignore the dumb comments above, they dont make sense for an engine analysis.

You should really use that membership to sharpen up your chess...

Also, after 20. Nc6??, 20. ...Bxe7+ is simpler. The white king is in check, so the black queen can't be captured.

I think you mean Bxd2+, and yea I agree with that 

Sorry. I learned descriptive first.

1.) You could have taken the knight for free. +3 Points. Furthermore it is a strong knight in the center and after taking him, you also would have a pawn in the half of your opponent with control of d3 and c3.

So one could say you gave a winning position for an unclear position. That I would call a plunder.

2.) The following will happen after your move with the Bishop:

Nc6+ King move Ne3 and the pin is gone and also the possibility to lead in material. So you are just still a pawn behind.

Idk