Comment 7 for bug 1772859

So your system is obviously not an 18.04 default installation from scratch - otherwise you would have netplan.io installed and this folder and file "/etc/netplan/01-netcfg.yaml" will exist on your system.

There is no known way to me to install an 18.04 system without having netplan (initially) as default.

Did you have removed netplan manually from that installation?
Or is your 18.04 system and upgrade from an older Ubuntu release (like 16.04 or 17.10)?

If it's an 18.04 installation from scratch please provide the installation logs.
They are during the installation available at /var/log/*
and after the installation still available at /var/log/installer/*