Your pawn has to be on your 5th rank, not the 6th. The opponent's pawn has to make its initial move two spaces and end up next to your pawn.
https://support.chess.com/customer/en/portal/articles/1444779-my-opponent-made-an-illegal-move-
Why does En Pessant not work in this position?
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Can someone please explain why I was not able to take the white pawn in front of the king that he moved ? Thank you.