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.
$active = ($form->error != '') ? '' : ' fabrikHide';
echo "<div class=\"fabrikMainError fabrikError$active\">";
echo FabrikHelperHTML::image('alert.png', 'form', $this->tmpl);
echo "$form->error</div>";?>
.fabrikHide{
display:none !important;
}