High Sierra macOS upgrade from Sierra - howto
I clicked the app-store link "get macos high sierra" in [support.apple.com/en-us/HT208969], then [Open] in the app store, but the popup box said: "This copy of the Install macOS High Sierra.app application is damaged, and can‘t be used to install macOS".
Why is apple's own file "damaged"?
How can I circumvent this obstacle?