I have a sequence of photos that popped up in Moments and I want to add my own music. I’ve downloaded it to both iPad and Mac and cannot get that specific music to ‘attach.’ I can’t click on it on the Mac and on the iPad, it doesn’t show up as one of the available tunes.
Weird. It worked on the iPhone but not the others. Still looking for clues 
Is the track from Apple Music or have you purchased it through iTunes?
I found that only the latter works for me.
I have an Apple Music subscription and the track was downloaded to all three devices.