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.
Sorry, in exaple is my mistake, but in list i use 3 underscores.I didn't try the plugin but in general placeholder syntax is
{tablename___elementname} (3 underscores)