I pass level 9 but I don't know why the position is impossibl
Well, the two knights could have danced around for a bit before putting black in check. But crucially each knight changes the colour of its square with every move. If white's knight is on black and black's knight is on white, then they've each made the same number of moves, which means black moved last, which means black moved while in check and was still in check at the end of their turn which is impossible.