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

The actual Download for Fabrik V4.1 is not working at the moment. The subscription process is though. We will advise once it is fixed. Sorry for any inconvenience.