If you have a Mac running Sierra, go to the app store, download High Sierra, and then create a usb installer as described here: https://blog.trendmicro.com/clean-install-macos-high-sierra-includes-installing-macos-usb-device/
Delete the copy you have, empty the trash, then restart the computer. Also disconnect any external drives that may have a copy of the installer.And so I have done. But still no luck. Says just ”open” in App-store![]()