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.
Thanks @troester.In list plugins it doesn't make sense to use element placeholders because you can select multiple rows.
You can only use 'general' placeholders like {$my->id},{year} etc