Can I download High Sierra in the new Mojave App Store?
Good news, the Full High Sierra installer will now download from the new App Store.
1. Open Apple store and start searching for macOS High Sierra.
2. Get the mscOS High Sierra.
3. After finishing downloading, open it you will get error that installer is too old to run.
4. So to install macOS High Sierra you need a lower version of OS to directly install it or create a bootable device to install.