link to macos mojave is not working..

There is nothing wrong with the link.

Are you using Firefox, then don't, that is the issue.

Open Safari, click on the link below,

How to upgrade to macOS Mojave - Apple Support

Go to Section 4 and click on Get macOS Mojave.

You will be redirected to the App Store

it should say Get or Download.

If it says Open then the App Store has detected a copy of Install macOS Move.app

in your Applications folder, move it to the Trash, empty the Trash.

Connect your mac to your router via cable, restart and try again.

Please mention the error, your mac model/year also from which macOS version are you trying to update?

I'm not sure if you are following the below guide.

How to upgrade to macOS Mojave - Apple Support

Note: Please make sure you are using Safari to open the link to get macOS Mojave.