Why is pawn c6 is not winning here and what is the missed win for black? When I played a game on chess.com and then ran the analysis afterward, the computer said that the pawn move c6 was incorrect.

 [title "Black to play"]
 [fen "r1b2rk1/p1p2p1p/5p2/2bNq3/8/8/PPP1BPPP/R2Q1RK1 b - - 0 1"]
 [startflipped "0"]

 1... c6


I don't understand why the move is considered a missed win. Can anyone explain why?
Look at the diagram. The engine tells you the best move (Rd8). 

The idea behind that move is that it pins the white knight to the queen and at the same time attacks the knight a second time. 


Moving the knight white would lose the queen (for a rook).
If white tries to defend the knight (with c4, Bc4 or Bf3), black would play c6 attacking the pinned knight with a pawn.


So the idea is: first pin, then win the knight with c6.

If you play c6 first, white could just retreat the knight to c3.

It is worth memorizing this theme (pin a piece, then attack it again) as it is quite common. Also works with diagonal pins by queen/bishop.