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.
Same here: https://fabrikar.com/forums/index.php?threads/registration-problems.53356... I can't login on the site, neither create new registration to download new version... site throws me a message:
Notice
Could not execute: /usr/sbin/sendmail