you’re on the right track – this is a permissions issue.
you’ll need to make sure that the plex user can read and write in the folders.
you might want to double-check the approach you’re using to mount the drive. If you mount it during bootup using /etc/fstab you might easily end up mounting it for root.
if you want it to work for the plex user as well the example below should help. Also, many of the Linux gurus recommend not to use the /media folder to mount your drives as this is used for removable media so the OS could get in your way:
This will mount the SMB network drive at //192.168.178.51/video in your local file system to the folder /plexmedialibrary/video with plex as its owner.