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.
Scheduled tasks are activated by traffic to your website. That is the only way they will run at the selected interval.