Lets say that /var/www/html is the web root in question. It is owned by root.root. You go to extend a frontpage web in to it and on the admin form you put apache.apache as the user.group. You get the error. Why? The reason is that you are saying to run as apache.apache but apache.apache has no ownership of the web root and can't create any files there. So the fix is to "chown apache.apache /var/www/html" and then the extend will work.