To enable the administrator review for a new user account:
1st way: please use this plugin https://wordpress.org/plugins/eonet-manual-user-approve/
2nd: https://wordpress.org/plugins/new-user-approve/ (but the message appears from the plugin, for now, so not possible to edit it).
3td way [PRO only]: While using the Ultimate Member plugin you could
Just change the Success registration message from
Registration was successful. We have sent you an email with your login information. Please use them to log into your account.
to something like:
Registration was successful. We have sent you an email with your login information. After admin review and approval you can log-in. You will be notified about the review results by email.