BBB Set Up to Network Boot on Ubuntu 18.04

I’m trying to setup network boot per this wiki:

I’m using the USB0 method, so I’m using step #9 instead of step #8.

However, my host PC is Ubuntu 18.04.

When I boot the BBB, while pressing the boot button, no SD card inserted, all I see on minicom is:

I can’t tell if these instructions are simply not valid for Ubuntu 18.04, or if I’m doing something wrong. Any advice or suggestions greatly appreciated.