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.
This thread is 2.5 yrs old and refers to Fabrik 1.x.
Unless you really are using 1.x, can you post in the appropriate 2.x forum, and give us some more details to go on?
-- hugh