Apple - 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

  1. Command - R: Install the latest macOS that was installed on your Mac.

  2. Option - Command - R: Upgrade to the latest macOS that is compatible with your Mac.

  3. 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.

You need to first make sure that your model of Mac supports running macOS High Sierra. If your Mac came pre-installed with macOS High Sierra of any earlier version, it can run macOS High Sierra.

Assuming it does, you can't run macOS High Sierra installer from within macOS Catalina (or any newer release of macOS). The error message is conveying precisely the same thing to you:

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

To downgrade your Mac by installing an older version of macOS, you need to create a bootable macOS installer on a removable media. Instructions for creating a bootable macOS installer can be found here:

  • How to create a bootable installer for macOS

Once you have the installer ready, restart your Mac with the removable media plugged in, press and hold the Option key, and select the installer on the boot screen. Once you are booted with the installer, you can either erase your disk and perform a fresh installation of macOS High Sierra, or partition your disk and install macOS High Sierra in parallel to your macOS Catalina install.