The Mac you are using has to be compatible with the macOS version you are trying to download.
We could not complete your purchase. This version of macOS 10.14 cannot be installed on this computer.
I STILL can’t download Mojave because the Mac I’m trying to download it on is not compatible. All I want to do is download macOS Mojave! I do understand why Apple did this, they don’t want a user to think they could install Mojave on a system that can’t run it. Apple should take this one step further and not show it as available in the App Store. Or have the button say Not Supported.