How can I install Mac OS Mojave. The link to the app store doesn't work.

That would be because you can install Catalina. If that is so, then you must install Catalina. Mojave is no longer supported on your model.

How to upgrade to macOS Catalina.

Before You Upgrade macOS

1. Fix any problems before upgrading.
2. Run First Aid in Disk Utility while booted from your Recovery HD.
3. Backup. Make a bootable clone using CarbonCopy Cloner or use TimeMachine.
4. Be sure your startup disk has 18-20GB of free space during the installation.
5. Upgrade third-party software for compatibility. See App Compatibility Table - RoaringApps. If you have mission critical software, then DO NOT upgrade until you know that software is compatible with the version of macOS you are about to install.
6. Disconnect all third-party peripherals.
7. Download the installer which will be in the Applications folder. Quit the installer after it opens, then make a copy of the installer and save it in the Downloads folder. The one in the Applications folder will be deleted after a successful installation.
8. The download is large, anywhere between 5 and 9GBs. It takes time to complete. Be patient.
9. Double-click on the installer in the Applications folder. Click on the start button to begin the installation.

The link does work, what you need to do is stop using Firefox and use Safari to click on the link,

How to upgrade to macOS Mojave - Apple Support

Go to Section 4 and click on Get macOS Mojave

this will redirect you 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'.

Have you tried this link? How to upgrade to macOS Mojave - Apple Support

Specifically the App Store link in step 4.

This is what I used and it works just fine.

Nope.

The error message produced by following the App Store page for installing macOS Mojave reads:

This copy of the Install macOS Mojave.app application is damaged, and can‘t be used to install macOS.

Ah, Eau Rouge. I see that I spread my frustration to you and for that I am sorry.

Solved now, thanks to your note about the possibility of a previous download preventing the App Store from enabling subsequent downloads. That was the problem; I had a download of 10:14 that had failed quickly, and I had mistakenly forgotten it existed. So despite my following all of the other steps before I ever posted to the forum, I was stuck until your tip about searching my Applications folder.

Thanks again. Peace.

Henry you are at it again. I have explained how to get around that error message in another thread you have decided to attach yourself to.