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
Please see our announcement here.
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?