When I click "Download" Mojave installer, I get error message "This copy of the Install macOS Mojave.app application is damaged, and can‘t be used to install macOS." Intent to upgrade from Sierra 10.12.6; Catalina loses some apps, Mojave won't run.?

Hi,

Open your Applications folder and trash "Install Mojave..." then download it again, using Safari. If it doesn't work, try booting into Safe Mode: How to use safe mode on your Mac - Apple Support and see if the issue persists there.

If you get a corrupted message...

Naw, not damaged, it's just that you have an old copy already in Applications, there are 2 cures...

Disconnect from the Internet & set the Date back to 2014, then you can run the Install MacsOS Mohave App in Applications, so expired Certificates are not expired.

Or... trash the Install MacsOS Mojave.app in Applications, empty trash & restart, then a new copy of a more recent version can be downloaded..

You're welcome.