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.
Joomla3.9.6? or 3.9.16?It seems Joomla doesn't display the library version correctly, have a look at libraries\fabrik\fabrik.xml to see your current version.