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.
How did you update? From which version to which version (Joomla and Fabrik)?is missing after update
If you can't find it in the plugin list (activated or not) it's obiously not installed at all, why is it "missing after update"?There is no plugin to activate for "Fabrik list - email", only "Fabrik form - Email"