Downgrading from macOS Catalina to macOS High Sierra I have macOS Catalina installed and I want to downgrade back down to macOS High Sierra. After I have downloaded the macOS High Sierra installer from the Mac App Store, opening it says:


  This copy of the "Install macOS" application is too old to be opened on this version of macOS


How do I install macOS High Sierra on my Mac?
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.