Is the amount you're trying to charge to the room equal to the full order value or only part of the value? If it is a partial amount (i.e. not the total cost of the order) you will not be able to charge to a room. It must be the full total so that the correct Revenue allocations can be accounted for against the payment.

Have you checked if the Opera/ Guestline interface is down? If it is you will need to restart it and contact your IT support team for initial assistance

Check in Opera that bookings have the No Posting box unticked