Was booking via Avianca (Lifemiles), but I don't think that was the issue because the inventory also disappeared from United.com at the same time. It seems to be the way TAP is managing its award inventory. In any event, it worked out fine for me since I only needed 4 seats. (My first attempt caused the initial problem, since I was hoping to book 2 seats each from two separate accounts -- that wouldn't work as explained).