Since you are already on the cruise, it does not matter. After reading your comment and other conflicting comments, i decided to try it.
I was able to log and make my Speciality Dining reservation with no charge. When I clicked to make a reservation, there was a notice starting that I had dining credits available to use. I selected my location and time. There was a summary with a charge, a credit applied to the charge, and it showed a balance of zero. If I recall correctly the system did ask for my charge card information, but after entering the information, the next step showed the summary that I stated.
I did notice once when I cancelled a reservations, I had to wait awhile until my credit showed up again.
i hope you are enjoying your cruise and you were able to reserve the days and times that you wanted.