Why cant I promote my pawn?

In one of my games I am trying to promote a pawn. Every time I move it onto my opponent's back rank it goes pink and will not allow me to promote the pawn. My timebank has now started running down on the game and I cant afford to time out on a game I have already practically won. Can anyone help?

you're in check

😞

Originally posted by trevor33
you're in check