Why for some music albuna Plex is available and for others isn’t
it available?
It depends on the device used.
Please refer to this webpage for more information as to why this is not working. In the past, Plex did mention that they are trying to change Plex Music, but right now, at best Plex Music is a beta product advertised as a premium and ready feature. There needs to be a complete overhaul to make it work properly.
The webpage:
https://support.plex.tv/hc/en-us/articles/205041208-Media-Server-Support-for-Premium-Music-Libraries
