You need to set the permission on the trans code folder so the Plex user has read/write access. If this is a mapped data-set, you may need to adjust the permissions on the data-set outside of your jail as well.
I don't recall all the details but if you search the forum you should be able to find a number of threads on the topic. HINT: In Google search 'site:forum.freenas.org "plex permissions" '