The error is referring to the permission of the directory, not the original file.
This would be the permission for your /includes directory, as it is attempting to write the backup copy
path: includes/
original file that you already have: config.inc.php file
backup file that is trying to be created: config-bak.inc.php
go to Control Panel Permission > Tools & Information: Permission Checks
and confirm that "includes" shows as a green "OK"
