I was setting up a mail server for a client and installation failed with following error.

Postinstall configuration returned an error.
No interface named addresses found

This was a Ubuntu 18.04 server rented from Contabo. Ubuntu 18.04 uses netplan on server side unlike Ubuntu 16.04 which uses ifupdown.

Solution is simple. Install ifupdown on Ubuntu 18.04 with following command.

apt install ifupdown

Then run Virtualmin installation script again.