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.
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