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
See Announcement
We have resolved the issue with the J! updater and this will be fixed in the next release.
What do you mean with "after selecting a record in the list"?
Export is always on the complete or filtered list (depending on your settings in the popup or the list definition), not on selected records.
Which browser are you using?
BTW: Have a look at your signature