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 have resolved the issue with the J! updater and this will be fixed in the next release.
Once you have installed Fabrik 3.x, if you want to stay up to date with the latest code updates between official releases, you can 'update from GitHub'. This will overwrite existing Fabrik files with the latest files available which often contain bug fixes and improvements.