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
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We have resolved the issue with the J! updater and this will be fixed in the next release.
DonePS. Would be great if the fix could be added to the core code for next release