SmartTubeNext shows two ways to create a new playlist, that I'm aware of. 1) ""Add this video to new playlist" by long-pressing on a video (before it plays) and 2) "Create playlist" by long-pressing on the Playlists section in SmartTubeNext's main menu. Neither of these methods work.
To confirm, this is a known issue? Is it a current limitation of the YouTube API? If so, why is the option shown?
The best thing you can do is go to github and ask/post issue report there if it's not there already.