Pink squares?

At the risk of posting a dumb question I have to ask why one of my games produces pink squares to indicate the move I have tried to make but the move is not sent; the game simply shows the pink squares and no register of having submitted the move.

Any thoughts?

It means you cannot make that move. It might be your king is in check position, so you must do something about that instead of the move you intended. It might also be the case you were moving a piece to a location it cannot be moved to. Like moving the knight from a white square to another white square.