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.
It seems fabrik is already in the database.
Check in #_extensions and delete all records with name = %fabrik%
(Backup the table before)
Do you have Kunena installed?