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.
Upgrading what? Which exact Joomla and Fabrik version are you running?this was prior to upgrading to 3.2