The link to download macOS Mojave 10.14 doesn't open.

The link to download Mojave does work.

Follow these instructions.

Please use Safari to access the following links do not use Firefox

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 Mojave download page

it should 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'.

Well as you can see from the screenshot it does open to the Mojave download page.

Have you tried restarting your mac and then opening Safari and trying the link again.

you could also try using Chrome if you have it installed, Firefox definitely does not work.

you also have a download happening in the back ground that could be interfering with the

connection to the App Store.

Is your mac working normally.