BA's search engine does not show LATAM availability, since they are not in OW. The seats, if available, are bookable on the phone (BA, as I've tried there, December 2021).
To search for availability I think you can try Delta's site, just pay attention on the operating airline.

In the last week LATAM stopped offering award seats to their own members (they just book whatever revenue seats are available at a fixed point to cash ratio), so not sure how this translates across OW.