Sometimes we do have issues with failed login attempts and it would be helpful to notify a supplementary staff member if this happens. However, despite being an admin myself, I’m not able to add another subscriber.
It seems most like that the query right above line 169 of /modules/System Admin/notificationSettings_manage_edit.php is returning no rows for some reason. Can you confirm this by adding the following to line 169:
echo $resultSelect->rowCount();
That is line 169 in v19.0.00, so might be different in earlier versions.
This SQL statement checks access to the relevant action, which in this case is Manage Users.
Maybe no other users have access to this in your system?