The basic groups for organizing playlists are free, if you want extended features like Multiselect you should become a patrons. Watch the full tutorial: https://www.youtube.com/watch?v=HpOt99xYGA4 It takes time to update the extension every time Youtube changes internally.
Furthermore, the decision to keep some features for the Patreon version of PocketTube was made to ensure the long-term feasibility of the free version.