The Plex Media Server is running as a separate (system) user account… on Linux that’s by default plex (in group plex). That user must have permission to see/access the files you’re trying to add to your library.
IIRC permissions inside your own user’s home directory can be tricky. That’s one of the reasons why I’ve opted to store my media on a separate folder in the „main“ file system (e.g. /plexmedia/music); that also made my life a bit easier when I started adding/mounting media from my network share.
ChuckPa‘s Linux tips are usually a good starting point to address such issues.
