Several years ago, I activated plex via paypal/microsoft store. Recently having to reinstall windows 10 and plex, the app is now requiring activation, but when I log in I get the following:
The Plex client app in the Microsoft store was discontinued several years ago. It is probably not fully functional even if it is possible to activate it somehow.
Instead, use Plex for Windows (download, documentation). Free. No Plex Pass or activation required.