"Playlist can't be added to your iCloud Music Library". Why?
"Playlists that reference other Playlists can not be uploaded"

It took me awhile to figure out the solution to this problem, but I eventually did. You can no longer just drag and drop songs from Apple Playlists to new ones that you create yourself, but you are still able to do it a different way. You also used to be able to just right-click on a song, a menu would pop up, and you could then "add to selected playlist" - this no longer works either. 

Solution: if you go into any one of your Apple Music playlists, hover over any one of the songs, you will notice a little icon with 3 dots show up - if you LEFT-click on this, a menu will pop up (different than the old menu), and then add the song to your desired new self-made playlist. This seems to work.

The problem is that iTunes still has both the old method and the new method active, but only the new method actually works.