I dealt with this leading up to New Years. They kept showing availability but throwing errors. Then after several days a bunch would become available. Then it looked like sold out (I was doing price matches for multiple dates so had to book on marriot and also on Priceline) I eventually was able to book multiple reservations-I am not sure how many reservations I had, it was a lot as I was running different combination of nights and made a few mistakes on my price matches so had to redo several (this was my first price matching so had to learn some tricks)-all told maybe like 20 reservations. It kept the same pattern for a week or two, Then they had no availability for a couple weeks then restarted this same pattern again, and then a week before I went they dropped prices by like 40% and had availability for several days. So yea…the best I could conclude is that it was a place holder until they were ready to officially release the nights available-for some reason. when I cancelled reservations they would occasionally became available within an hour or two, but most of my cancellations were not available as far as I could tell-in fact the whole room category would show fully booked for several days until the above pattern started again. Occasionally they would remove the refundable options for several days and then bring them back, or block out all queens and have only kings and then switch it again a week later. For a couple days every room was showing 1k plus only one available.
mall of these things happened on both marriot.com -and Priceline though it would s9metimes show up later- with the rooms usually showing available on both sites before they actually became available.
this was different than when Priceline showed 1 available on Priceline with a good VIP deal but was not showing up on marriot. In those cases even when I purchased the last room myself it would stay on Priceline for HOURS-sometimes even into the next day. Out of curiosity I tried to book it again several times using different devices, accounts etc. and it would throw that same type of error until it was finally removed.
Basically, this information doesn’t help any beyond saying that it could mean anything, I bet there will be availability if you keep checking, don’t let the error convince you it will be hard to book a night and commit to a bad deal.