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
Please post subscription issues here
We have resolved the issue with the J! updater and this will be fixed in the next release.
This post is about the list plugin (to send an email to one or several people displayed in a list) while you seem to refer to the FORM email plugin (to send an email when you are submitting a form).
If so, check the Wiki : http://fabrikar.com/wiki/index.php/Form_plugin_email