The link does work, but when you get there I bet it says' Open' not 'Get or'Download' as it should.
The error message actually refers to the App Store detecting a copy of the
Install macOS Mojave.app in your Applications folder.
Go to your Applications folder locate Install macOS Mojave.app and move it to the Trash.
Empty the Trash.
Connect your mac to your router via cable rather than rely on WiFi.
Restart your mac.
Open Safari,
Click on this link again.
How to upgrade to macOS Mojave - Apple Support
Go to Section 4 and click on Get macOS Mojave
You will now be redirected to the App Store Mojave download page.
It will now say either 'Get' or 'Download' instead of 'Open'
Do not use the mac while it is downloading and do not use any other
devices that share the same internet connection while your mac is downloading.
