This copy of the Install macOS Mojave.app application is damaged, and can’t be used to install macOS

Go to the Applications Folder. Find "Install macOS Mojave. app" and drag it to the Trash. Enter your administrative password to trash it. Then download a fresh copy of the Mojave installer and run it.

How to upgrade to macOS Mojave - Apple Support

Did you get the link to the update from that page: How to upgrade to macOS Mojave - Apple Support ?

Check if there are outstanding updates for your macOS 10.13. Especially security updates.

WD

its about certificates... read this https://osxdaily.com/2019/10/24/fix-install-macos-application-damaged-cant-be-used-error-mac/

Yes, this is related to my answer about Security Updates above. Sometime it works. Others like suggested in the link you posted, were successful by playing with the Mac System Date.

WD