What version of PMS are you running? I’m on 1.41.8.9834, the current public release.
Are you using the Plex Movie scanner and agent (the default) or the legacy agents/scanners? Edit the library → Advanced to see the options.
After my first reply I ran across this long thread: Plex Agent no longer working and Add to Watchlist option is also now gone - #278 by X_4357
Others are reporting the same problem, but root cause is unclear (from what I can see anyway).
