Can someone explain why e5 is mistake???

Because it loses a lot of material: 18. Bxe5 fxe5 19. Qf6 and the rook drops. But since you'd still have a slight material advantage (bishop pair + 2 pawns for the exchange) and white's position is otherwise terrible (his king is unsafe and he'll lose instantly if he tries castling queenside) it's only a mistake and not a blunder.

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