You are accessing Plex web through your local server (PMS 1.19.1 has Plex Web 4.22.3 installed locally). This version of Plex Web does not yet support the new approach to handle smart playlists – so as per leelynds post… you should still be able to create a smart playlist by creating a custom filter and create a playlist from there.
If you use the hosted version of Plex Web (through app.plex.tv/desktop, this will give you the new/updated smart playlist management/handling
Edit: editing of smart playlists and the related changes to the UI have been added as experimental features of Plex Web 4.31.1.
See below the related release announcement.
