I can't access my Music Collection to add music to a slideshow in Photos.

You can select songs from your Music Library, but they have to be downloaded from iCloud, if you are iCloud with Music (for example iTunes Match). I am always playing the songs in Music to force a download from iCloud, before I try to add them to a slideshow. and you can oly use songs that are not protected. Is the system version given below your question still valid? Apple has changed the rules which songs were allowed between the system versions. On some versions we could use any song without digital rights, on some versions only the theme music and Apple Music.