There are certain playlists that can’t be updated by Playlisty because Apple Music says they are “read only”.
The usual reason for this is that you’ve marked the playlist as “Collaborative”. Unfortunately Apple doesn’t currently allow third-party apps such as Playlisty to update collaborative playlists.
Smart playlists, playlist folders and playlists owned by other users which you’ve linked to your library are also usually marked as read-only.