Plex is unable to write to the database.
Check ownership / permissions for the Plex data directory.
The last statement in the log snip you provided mentions database corruption. However, that might be due to the access problems.
I do not run truenas, so cannot help with specifics on correcting the problem. I searched the forum for “attempt to write a readonly database” and found a similar post from 2018. Hopefully it will get you pointed in the right direction: Plex won’t start/
https://support.plex.tv/articles/202915258-where-is-the-plex-media-server-data-directory-located/
