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.
I confirm the 'echo' command in 'eval populate' works with the version 3.10Echo from eval populate should never have worked. The eval populate must return an array of options. You will need to insert the constant another way. I have used sourcerer to include things like this in the forms intro.