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.
[04-Mar-2011 16:25:17] PHP Fatal error: require_once() [<a href='function.require'>function.require</a>]: Failed opening required 'C:\Program Files\Zend\Apache2\htdocs\joomla_16\plugins\fabrik_element\databasejoin\databasejoin.php' in C:\Program Files\Zend\Apache2\htdocs\joomla_16\plugins\fabrik_element\user\user.php on line 13