Why Nxd5 is a mistake?

I don't think it is not playable, there just are better alternatives. I used Decode Chess to gather the following:
Before Nxd5:
Black has a decisive advantage (-3.48) but should beware of White playing e5
After playing Nxd5:
Black has a big advantage (-1.15); White should beware of Black playing Ne3
I hope this helps shed some light on this position.

Winning the bishop is the goal, but c6 also saves black’s bishop by giving it a square to escape to and avoids capture after e5.

instead the move c6 would have been better as you would win a piece, the bishop would have no control or flank squares to go to and you would be up winning the game eventually. i hope that helps but yeah its hard to spot these subtle mistakes or inaccurate moves in games friend, i think really just keep trusting your gut and take your time its just destiny