catalinadowngradehigh 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?
Massive bump to @IlyaB, I was stumped trying to get the full 5gb Install MacOS High Sierra.app but was stuck with the 19mb stub version.
My Process to get a working app that is currently making my bootable USB for High Sierra
1: Go to Mac App Store and download High Sierra (the 19mb stub)
2: Click Continue in the installer app once it downloads
3: Choose your boot disk (or any disk that has space)
4: Allow the installer to download (this is the contents of the missing "SharedSupport" folder within the installer stub)
5: The installer will prompt you to restart after finishing the 5gb download, at this point quit the installer and do not restart your system!
6: Go to your root (or whichever location you chose in step 3) and find the folder "macOS Install Data"
7: Copy the contents of that folder disregarding .DS_Store 
(9 files, folders, dmgs, pkgs, and plists)
8: Go to your Applications folder where the Mac App Store downloaded the stub version of "Install macOS High Sierra"
9: Right Click on that file and "Show Package Contents"
10: Within the Contents folder, create a new folder called "SharedSupport"
11: Paste the files you copied earlier from the installer download
12: And you're done! You should have a fully functional Installer file for bootable USB drives or just copying to other systems whether they be offline, or you just don't want to download the installer over and over.
I am currently in the lengthy process of creating a bootable USB drive to make a Coffee Lake i7 8700k Hackintosh build. I'm following this guide with the recommended motherboard. I'll try to update here with my success/ failure.
Given the context of your question, it's unclear if you re-downloaded the installer from the App Store after the first failure or if you ran the upgrade application again. Yes, your computer is supported for the High Sierra update. I have a few suggestions:
If the problem persists, the solution will probably involve something more drastic. If you have an available flash drive, try making a bootable flash drive installer. Restart and hold (Option) and select the bootable USB disk and hit enter. Proceed through the install process. Notify us of the result and we'll proceed from there :)
Good luck ;)
Best Answer
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.