why did I fail this puzzle

Playing a5+ is a nice idea. However, you evidently haven't learned about en passant captures.
You forgot about en passant.
I faced a very similar situation many years ago (a similar pawn push, which is "checkmate") in what would be one of two occurrences of me finding out about en passant, where the other occurrence is in a real-life tournament where I panicked upon seeing an en passant capture early in the opening.

You forgot about en passant.
I faced a very similar situation many years ago (a similar pawn push, which is "checkmate") in what would be one of two occurrences of me finding out about en passant, where the other occurrence is in a real-life tournament where I panicked upon seeing an en passant capture early in the opening.
I can understand that feeling. Even when you know about the rule, but you are lower rated it is possible to completely miss that move.