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.
Check in your form's settings that you have selected a template (in the layout tab). If it is set to "default", it will set an error as there are no more a "default" template in Fabrik 3.1b.