There are usually two possible reasons for this error message:
- the primary database file of Plex server is heavily damaged. Try to either repair it https://support.plex.tv/articles/repair-a-corrupted-database/ , or replace it with a backup copy https://support.plex.tv/articles/202485658-restore-a-database-backed-up-via-scheduled-tasks/ .
- the location of the Plex data folder has previously been moved to a custom location, which now is either inaccessible, or no longer existent. If you did this, take a look into the Windows Registry to see at which location Plex is expecting to find its data. See [HowTo] An extended guide on how to move the Plex data folder on Windows for details.
