Downgrading from macOS Catalina to macOS High Sierra
Did you try the following Recovery key combination as in your case Shift-Option-command-R may help 


Command - R: Install the latest macOS that was installed on your Mac.
Option - Command - R: Upgrade to the latest macOS that is compatible with your Mac.
Shift - Option - Command - R: 
 Install the macOS that came with your Mac, or the closest version still available.


See if your High Serria is still available from it. I think it should be as you can still get it from App Store.