https://support.plex.tv/hc/en-us/articles/202934883-Cinema-Trailers-Extras
Edit your library, go to the ‘Advanced’ tab and verify that ‘Enable Cinema Trailers’ is checked
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Have you refreshed your movie library after you activated your Plex Pass?
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The trailer feature needs the ‘Plex Movie’ agent.
If you used The Moviedatabase agent to ‘match’ your movies you need to do this:
Settings - Server - Agents - Movies - TheMoviedatabase
verify a checkmark is placed on the line ‘Plex Movie’
Afterwards, refresh your movie library -
If you used 3rd party metadata agents, like the .nfo importer or the one from MediaMaster, you’re out of luck.