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.
On the update page, when I check for new updates, I get a messge zeta to 4.0 and in the ftp, I have two entries in the manifests/packages for "fabrik" and "fabrikbase" (see attached picture)."Refresh cache" on all Fabrik entries in Manage / Extensions