Apple's October 2019 link to download 10.13 High Sierra failed:
https://itunes.apple.com/us/app/macos-high-sierra/id1246284741?ls=1&mt=12
Got this popup:
"This copy of the Install macOS High Sierra.app application is damaged, and can‘t be used to install macOS."
MacBook Air
This usually refers to a partial download of High Sierra, look in your Applications folder for Install macOS High Sierra.app
move it to the Trash, empty the Trash.
If you can, connect your mac to your router using a cabled connection. USB to ethernet cable or Thunderbolt to Ethernet cable.
Restart your mac, click on this link, How to upgrade to macOS High Sierra - Apple Support
Go to Section 4 and click on Get macOS High Sierra.
Do not use your mac while it is downloading. If any other devices share your internet connection, don't use them either.
This usually refers to a partial download of High Sierra, look in your Applications folder for Install macOS High Sierra.app
move it to the Trash, empty the Trash.
If you can, connect your mac to your router using a cabled connection. USB to ethernet cable or Thunderbolt to Ethernet cable.
Restart your mac, click on this link, How to upgrade to macOS High Sierra - Apple Support
Go to Section 4 and click on Get macOS High Sierra.
Do not use your mac while it is downloading. If any other devices share your internet connection, don't use them either.
It also may just mean an old good download where the Certificate expired...
Disconnect from the Internet, set the date back to 2014 or 2015 & try again.
Other option is to delete Install MacOS High Sierra in Applications folder & download a new copy.
"This copy of the Install macOS High Sierra.app application is damaged, and can‘t be used to install macOS."
