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.
Yeah, only some element have the Hidden option.
You can try (although this is unofficial) edit the element settings, switching the type to "field", set Hidden, then switch type back to the original type, then save.
-- hugh