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.
/** @var string Product */
var $PRODUCT = 'Fabrik';
/** @var int Main Release Level */
var $RELEASE = '1.0';
/** @var string Development Status */
var $DEV_STATUS = 'Stable';
/** @var int Sub Release Level */
var $DEV_LEVEL = '6';