This copy of the Install macOS High Sierra application is damaged, and can‘t be used to install macOS

Open your Application folder, locate Install macOS High Sierra.app, move it to the Trash, empty the Trash.

Connect your mac to your router using a cable rather than rely on WiFi. Restart your Mac go back to the App Store High Sierra

page and press Get. The download should start, do not use your mac while it is downloading and if any other devices share the same internet connection don't use them either.

disconnect your clock and set the year somewhere between 2015-2017

then launch the installer again.

https://support.apple.com/guide/mac-help/set-the-date-and-time-mchlp2996/mac

Did you get the OS installed, if so you should reset your Date & Time back to today.

Did you disconnect from the Internet before running it?

Or... trash the Install MacsOS High Sierra.app in Applications then a new copy can be downloaded.