Plex Mix not showing up for any album



I can't find any way to use Plex Mix in the android app. It doesn't show up even for "popular" albums. I've got a lifetime subscription plan.

Android 8, Nexus 5x, App version:
Ubuntu server, App version:


It seems that I had to remove my whole music library and re-import it as a 'premium' library.

Maybe there is (or certainly should be) a way to just 'upgrade' a library but I couldn't find it