Sierra to Mojave

I haven't upgraded in quite a while. I'm still using 10.12.6. How do I go from Sierra to at least Mojave? I'm guessing that in order to get to Mojave I need to upgrade to High Sierra first. When I click on High Sierra in the App store, I get this message:

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

I managed to download 10.13.2 but when I attempt to install it tells me that I need 10.13, but as I said I can't get it installed.

I also found a download section on Apple's site, but no download for basic 10.13. It goes directly from 10.12.6 to 10.13.2 - there is a missing download for the main version of 10.13.