What are the rules on this app's games?
You can't take kings. As to the other move, what game and move number?
I solved that issue about the king but the moves are usually when I can prevent a check or other taking of a piece. For instance, in my last game a rook was able to take the king but put itself in the line of my bishop. But it wouldn't let me take the rook and was forcing me to move the king out of the way, when you should be allowed to move any piece that stops the check. My games keep losing because I set up the opponent in traps only to be stopped from moving pieces where they should be allowed to go.