Ok I had this same problem and I finally got High Sierra to install. Here is what I did.
Step one you will need to download a copy of the high Sierra dmg file.
macOS.High.Sierra.10.13.6.dmg
Step two you will need to download the High Sierra Patcher tool
here
Step Three on the current Mac you will have to allow the patcher app to install for use through the security settings
Step four connect a blank thumbdrive to your Mac to install the High Sierra OS to needs to be over 16gigs
Step five run the High Sierra patcher program and point it to the .DMG file you downloaded earlier and tell it to install it on the thumb drive
This process takes some time to complete and failed the first time I tried it so try again. The second time the process finally completed.
Step six On the old Mac start the Mac while pressing command option and R
Step seven make sure the this Mac is not connected to the internet via wifi
Step eight go to utilities at the top of the screen and open the terminal app
Step nine type the command
date 0101010118 and press enter
this will set the date and time on this Mac.
Step ten close the terminal app, connect the usb stick you made to this Mac, and then restart the Mac while holding the option key.
Step ten select the thumb drive that has High Sierra operating system installed on it and go through the installation process.
So far this is the only way I have been able to get this to work. I ran all of the initial processes using my M2 MacBook Pro 13" so it should work for all of you if you need to create a usb Mac OS disk.
Hopefully this helps you guys it took me forever to get this to work!