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.
You did.I have not done it to form
I think this is because you are using "{gestion___Operador}" as the "From" ("De") setting. And as the tooltip for that says, it has to be an email address. But Operador is a user element set to use "Name".
In the email plugin setup you must use elements/placeholders which contain a valid email address.You must make sure that your "From" field/placeholder contains a valid email address.
e.g. use an additional user element set to show the email