How To Downgrade macOS High Sierra

1. Start up from macOS Recovery

To start up from macOS Recovery, turn on your Mac and immediately press and hold one of the following combinations on your keyboard. Command-R is generally recommended, especially if you never installed macOS Sierra 10.12.4 or later… https://support.apple.com/en-us/HT204904#notupdated

Command (⌘)-R

Install the latest macOS that was installed on your Mac.

Option-⌘-R

Upgrade to the latest macOS compatible with your Mac.

Shift-Option-⌘-R

Install the macOS that came with your Mac, or the closest version still available.

Or...

Do you have a Time Machine backup to Restore to/from?

https://support.apple.com/guide/mac-help/revert-to-a-previous-macos-version-mh15216/mac

Or333

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

Any luck yet?