Would be helpful if you showed us the game where it happened. Most likely case was that you couldn't take the pawn, perhaps because it was defended and you were trying to take it with your king, or the piece you were trying to capture with was pinned to the king and hence couldn't move.
#1 You might be able to understand why you can't take the pawn...if you did your own thinking... and not let a engine do your thinking. :[