I followed a link provided by Apple support in March 2020 to access the macOS Mojave 10.14 software. When I click on the link I get an error that says, "This copy of the Install macOS Mojave application is damaged and can't be used to install macOS.

The link works fine.

The message means that the App Store has detected a copy of Install macOS Mojave.app

in your Applications folder or elsewhere on your mac.

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

Connect your mac to your router via cable,

Restart your mac and go back to the App Store Mojave download page,

it will say Get or Download, not Open as before.

do not use the mac while it is downloading or any other devices that

share the same internet connection.

That poorly-worded advisory mean the installer you just downloaded found an older version of the installer in your Applications folder. Trash the old one, then things should work.

Use Safari for any system downloads; Firefox is not woking properly with those at the moment.