Note that su - apt-mirror -c apt-mirror will not work on Ubuntu if you have not set a root password. But sudo apt-mirror -c apt-mirror will always give:

apt-mirror: invalid config file specified at /usr/bin/apt-mirror line 101.

There is almost NO information regarding this error but found that the ArsGeek  Blog uses sudo apt-mirror /etc/apt/mirror.list which works perfectly! Only way I could start downloading the repo's.