So, the machine auto-activates Windows 10 Home Edition at install time. No prompts, no nothing. You insert the generic upgrade key, and it throws an unhelpful error message back at you:
This probably surprises nobody, but trying again, as the above message suggests, will not work for you.
The solution? Stuff the install media (in my case a USB stick made with the Media Creation Tool), back into the machine, then run the following command:
setup.exe /auto upgrade /pkey VK7JG-NPHTM-C97JM-9MPGT-3V66T
There’s that generic upgrade key again.
After it’s done updating, proceed as usual.