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.
Do you know, by chance, if disabling mootools is going to cause errors with Fabrik?
The question is how difficult is it to migrate a complex Joomla 3/Fabrik/Gantry site to J4/F4?