Why was en pessant available?

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Avatar of Justified444
Can someone please explain why I couldn't use en pessant at move 18? [Event "?"]
[Site "Chess.com iPhone"]
[Date "2025.10.21"]
[Round "?"]
[White "Referee1988"]
[Black "Justified444"]
[Result "0-1"]
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]
[WhiteElo "999"]
[BlackElo "1004"]

1. e4 Nf6 2. Nc3 d6 3. Bc4 g6 4. d3 Bg7 5. Nf3 O-O 6. Bg5 h6 7. Bxf6 exf6 8. O-O Nc6 9. h3 a6 10. a3 b5 11. Ba2 f5 12. Re1 fxe4 13. dxe4 Bb7 14. Bd5 Rb8 15. Bxc6 Bxc6 16. a4 b4 17. Nd5 b3 18. c4 Bxb2 19. Rb1 Bg7 20. Rxb3 Bxa4 21. Nd2 Bxb3 22. Nxb3 c6 23. Ne3 a5 24. Qd2 a4 25. Na1 a3 26. Qa2 Qa5 27. Kf1 Qc3 28. Nec2 Qb2 29. Qxa3 Qxa3 30. Nxa3 Ra8 31. N3c2 Rfb8 32. f4 Rb2 33. e5 dxe5 34. fxe5 Rba2 35. Kf2 Kf8 36. Kf3 Ke7 37. g4 Ke6 38. Ke4 Bxe5 39. Kd3 Kf6 40. h4 Bxa1 {0-1}
Avatar of blueemu
If your Pawn had been on b4 (not on b3), then you could have captured en passant.
 
Avatar of Justified444
Thanks!
Avatar of Vanillaice5298
Because if 2 pawns are on the 5th then the person moving can capture only on the first chance