Mojave upgrade on app store is damaged and can't be used to install

I have 2012 macbook air that is running High Sierra and want to upgrade to Mojave.

But when I navigate to https://apps.apple.com/us/app/macos-mojave/id1398502828?mt=12 and click on 'open' I get a message that says that the copy of mojave is damaged and can't be used to install it.

What is going on?

I have software for my analytic work that will not work under any of the newer systems.

Please, anyone, can you advise me?