My guess is that the API has been deprecated. It used to return some mood playlists.
I'm looking forward to having an official response from Apple...
Here are the ids we were using in our app:
976439503 - chill
976439525 - fitness
976439521 - focus
1142652618 - throwback
1142652615 - motivation
976439518 - romance
1442122106 - sleep
976439500 - blue
976439512 - feel good
976439514 - party