You will get that or another error always with the TPB tracker now, since it is gone. You can ignore it. Use the magnet link to load torrents from TPB. This aspect of the torrent community is in flux right now. The usual practice of registering a torrent with a particular tracker is particularly in transition. The openbittorrent.com tracker is one where registration is not needed, and indeed can’t be done.
You don’t have to do anything different, though. It means that you will not get the error ‘unregistered torrent’ from openbittorrent. All torrents that anyone inquires about are, therefore, ‘registered’ by virtue of having been inquired of.
Torrents from TPB should by preference by loaded with their magnet links instead. In some senses that’s more convenient, as the magnet link can simply be dragged & dropped onto BC. This would be a good area to pay attention to, as it develops.