This is a common problem with the older classics.
The patent/license holders are pulling them OFFLINE with DMCA take-down orders.
When those orders reach Plex’s provider, Plex loses access too.
My solution has been to go find posters I want.
I then place the poster in the folder with the movie. (Same name - different extension)
With that part done, Settings - [server] - Agents - Enable Local Media Assets. and lastly, Scan Files .
https://support.plex.tv/articles/naming-and-organizing-your-movie-media-files/
