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.
Thank you!
Can I assume that if I upgrade Joomla to version 5, I'll have to create the folder "Joomla root/administrator/components/com_fabrik/classes/50" and can upload the same FormField.php file to the new "50" directory?