It could be an App error. I would ask others on your roll call, if they’re able to book the CG.
Did you try DMW on the website version of the App? Go to your cruise personalizer >checkin>dine my way>sign in using your name, dob and booking no.
Not likely every day and time is booked. Does it even show CG as a choice, but just that it is not available?
If you can’t get anything booked,
I would report it to the IT team.