Hello! To move already existing playlists into a folder, go to 'Playlists' then on top of the 'Create Folder' button you have a 'Select' checkbox. Check it and then you can select which ever playlists you desire, once all the according playlists are selected, click 'Cut' in the bottom, then move into the folder where you'd want these playlists to be and click 'Paste here'. This will move already existing playlists into a folder!
