Fix for upgrade error (circle with a line through it)

This is what finally worked for me after multiple different attempts to do a clean install:

Upgrade from MacOS Sierra (you may be able to skip this and install from High Sierra boot installer directly. When I tried to install on blank HD from installer on external HD, I kept getting caught at the circle with a line through it error so I decided to do a clean install of Sierra and upgrade from that).

When you install the High Sierra upgrade, it will want to reboot (and this is where I kept running into that circle with a line through it on boot up).

When it get stuck on the circle with a line through it on reboot, shut down the computer by holding down the power button. Then follow these instructions:

"Here's how to reset your PRAM:

Shut down your machine. Yes, all the way down, not sleep or logging out.

Press the power button and then press command-option-p-r. ...

Hold those keys down until your Mac reboots again and you here the startup chime.

Let go of the keys and let your Mac reboot normally."

Source: https://thenextweb.com/lifehacks/2017/06/14/how-when-why-to-reset-the-pram-smc-o n-your-mac/