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.
* Custom code
*
* To use, copy this file to Custom.php and rename the class from CustomSample to Custom.
Thanks. It's not a big problem. But it's better when everything goes smoothly
Let's wait for Gamma version (don't know the greek "f" equivalent...)