Thank you for this insight. As an update (in case anyone cares), I ended up booking the fare through TAP's online chat. It took a while and I had to provide screenshots of the online fare and error message to get the $70 chat booking fee waived, but came through in the end. I did have to have 2 more chats to correct my dob (which was wrong the first time I checked my booking but when I was on chat the TAP rep saw the right year, so somehow it mysteriously corrected whilst I was on/waiting for chat) and then again to enter my United MP # (it seems that the first rep put all my info into a Miles&Go profile so it wouldn't let me select/enter my United # - I only figured this out because there was a small notation in my record that my info was inputted from my M&G account). The (3rd) chat rep was able to rectify this quickly.

Net, website did not work at all and, while I got the flights/fare I wanted, it took a lot longer and multiple follow-ups via chat. Chase it is next time.