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.
Is it working in a standard form (menu item, list add/edit...)?Did you check your page source with your browser console if it's "there" but maybe hidden via CSS or covered by the following element?