Castling queen side

Is it this game? White may not castle because the knight on b2 covers d1. You may not castle the king through check.

If an opponent's piece threatens any of the squares your king has to go through, or land on, you also may not castle, if that is not the reason perhaps you have already moved the rook or king, if that is still not the reason, then perhaps you were disconnected. If that still isn't the problem then it could be a one time glitch, you may want to try another game and see if it will work then. Also the king only moves 2 squares on the queen side as well. I hope this was helpful. Good luck

You can never castle if in doing so, you king moves through a square under attack by an opposition piece. The nice thing about computers is that they won't let you make a move if it is illegal.