This copy of the Install macOS High Sierra.app application is damaged
I have a mid2010 MBP that can only run High Sierra (10.13.x)
I am trying to install High Sierra via the App Store (as indicated via this page https://support.apple.com/en-us/HT208969) but I get this popup when I click the Open button.
"This copy of the Install macOS High Sierra.app application is damaged, and can‘t be used to install macOS."
since all the instructions seem to say that you need to download via the app store, I'm a bit stuck.