I'm at work so I haven't been able to test this, but it appears that you need to use "On Apple Music" to command Alexa.
This is from the support site:
Ask Alexa to play your music
You can ask Alexa to play songs, artists, albums, or genres from Apple Music. You can also play Beats 1 radio, playlists from Apple Music's editors, or playlists in your iCloud Music Library. Say something like “Alexa, play Pure Pop on Apple Music,” or "Alexa, play Bebe Rexha on Apple Music."
If anyone has tried this out, I'd love to know if I can just say "Play my Metal Playlist" or if I have to add "on Apple Music."