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
See Announcement
We have resolved the issue with the J! updater and this will be fixed in the next release.
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