Updating from Sierra to Mojave, whilst avoiding Catalina.
I want to upgrade my iMac's OS but want to avoid Catalina, which I seem to be constantly directed towards by Apple. I'm trying to find where to download Mojave. but the only place I found it doesn't have a download button, just one that says 'Open'. When I click on that I get an error message saying 'This copy of the Install Mac OS Mojave application is damaged and can't be used'
https://apps.apple.com/gb/app/macos-mojave/id1398502828?ls=1&mt=12
Where can I get a working copy of Mojave?
iMac 27" 5K, macOS 10.12