The iTunes channel is accessible on the Apple TV Plex app screen and correctly lists the music in the iTunes album but will not play any of it with the error in the title of this post.
Someone posted similar in the channels discussion a while back and received the comment that the iTunes channel wasn't quite a channel in the usual sense and questions should be asked on the main forum.
PMS Version 1.3.3.3148 (Up to date) running in FreeNAS 9.10 jail
PLEX app on Apple TV 3 (Up to date)