Why is this a mistake?

In move 17 why should the rook take the pawn? If it does can't the knight fork the white bishop and rook

After Ned7 (the fork) your knight can take on d7 and after that you move the rook away - winning the pawn. When you move the Knight, you don't win the pawn.

After 17. Rb6 Nd5 18. exd5 Nc4+ 19. Kf2 Nxb6 20. c4, White has 3 passed pawns, and black has no counterplay. After 17. Nd3 Nc4 18. Rc6 Be6, black has some compensation because of white's doubled c-pawns. Both are easily winning for white, but the latter position allows black more chances.

Thanks @jonesmh :)
Makes sense