Fresh-Installing That Old Mac
I'm getting a couple of old MacBooks ready to sell – many years after they should have been moved out of the house. (One of them I actually sent into Apple as a trade-in and they sent it back saying they couldn't access it. So weird, I had it all ready when I sent it. And sure enough I can't get logged into it either.)
Anyway, here are some a couple of articles that helped me get macOS High Sierra installed on a machine that for some reason could not get macOS reinstalled via the remote installation process:
The latter helps you set up a USB drive with a full version of the macOS installer, which in my case was the only way I could get macOS re-installed on a super-old MacBook.