That error message usually means that the App Store has detected a copy of Install macOS Mojave.app
in your Applications folder. Locate it, move it to the Trash, empty the Trash.
Connect your mac to your router via cable rather than rely on WiFi.
Restart your mac and go back to the App Store Mojave page, it should now say 'Get' or 'Download'.
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOS Mojave.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.