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.
As I already said, you do not want a user to be able to add elements in the frontend. Imagine "only" 1,000 users able to add "only" 5 elements each... that could result in 5,000 Fabrik elements being added. Imagine all the implications, and you should want to happily drop that idea.... the idea is the user add unlimited elements ... And allow user to add extra elements if they need to. Ofcourse this have to be donde front end ...