Bought a song from this NPC for 300gp, the song I received "Dirge of the Tormented" did not function, songbook said "[Songbook] This is not a valid song."
I got another song which did function so it may simply be that particular song doesn't work!
Did it add it to your book but wouldn't let you select it as your active song?
If so it may be because you don't have the feat "Curse Song". It is required to sing/activate this song.
I just had the identical problem, and my PC does have Curse Song (though I know it's not needed). It seems as if "Dirge of the Tormented" is likely bugged.