For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
See Announcement
We are still working on the Joomla update.
The plugin is not installed, see the attachment. But I dont know how to get the plugin?"Discover" is only if you are running a Fabrik GitHub update.
If you are running the downloaded Fabrik3.1rc2 (or Fabrik updated via Joomla updater) download and install additional plugins.
Check if it's installed but not enabled.