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.
<field name="email_template"
type="filelist"
description="PLG_FABRIK_FORM_EMAIL_TEMPLATE_DESC"
directory="/plugins/fabrik_form/email/tmpl/"
hide_default="1"
label="PLG_FABRIK_FORM_EMAIL_TEMPLATE_LABEL"
repeat="true" />