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.
foreach ($formModel->formData['eh_clients_77_repeat___client_name'] as $key => $value) {
$formModel->formData['eh_clients_77_repeat___client_name'] [$key] = $formModel->formData['maintable___mainelement'];
$formModel->formData['eh_clients_77_repeat___client_name_raw'] [$key] = $formModel->formData['maintable___mainelement_raw'];
}