Not quite the same as you; I was able to book with dining credits just fine (I don’t remember the wording exactly, but it is made clear during booking that you have dining credit available).
However, I later wanted to change to a different dining venue and after cancelling one of the bookings the credit didn’t return to my account and wanted to charge my card.
Customer Services said there was an IT glitch affecting a small number of customers which meant dining credits weren’t being added to their accounts properly, so possibly this is what you are experiencing. Unfortunately the suggested solution was to book (and get charged) and sort out once onboard.
In my case it seems I was just being impatient. A few hours later the dining credit had returned to my account and I was able to cancel the cash booking and rebook with my credit 🙂