When trying to upgrade from High Sierra to Mojave, the message said that the installer was damaged and Mojave can not be downloaded. What do I do now?

The certificate for Mojave is fine, if you get the installer is damaged message it means the App Store

is detecting a copy of Install macOS Mojave.app in your Applications folder.

Locate it, move it to the Trash, empty the Trash.

Connect to your router via cable.

Restart your mac, go back to the App Store Mojave download page it should now say

'Get' or 'Download' not 'Open'.

The download is 6.05GBs in size

Do not use the mac while it is downloading and do not use any other devices that share the same

internet connection.

I am having this problem also. The upgrade will download, but upon completion I get an error message saying that the copy of the Install Mojave application is damaged and can't be used. I have tried repeatedly, deleting the application after each fail. I chatted with Apple Support and they said that the problem is that the security certificate is not available for the update. It may get issued again but no timeline on that...could be hours, or much longer.

Typically, this message indicates that you are attempting to use an outdated installer. Are you trying to get this from the Apple App Store?

There is an older copy of the installer in your Applications folder. Delete it, then try again. Also use Safari for any downloads. FireFox seems to be having issues with Apple system downloads.

Thanks Eau Rouge, emptying the trash and restarting did the trick. Mojave installed with no problems.