On my iMac Late 2013 I have macOS High Sierra 10.13.6. Why can't I update to Mojave 10.14.6?

You should be able to. Open link in Safari : https://apps.apple.com/us/app/macos-mojave/id1398502828?mt=12

When I open that link or the one for Catalina (Currently on Sierra with a Late-2013 iMac) it only shows me MacOS Previews with no option to select or download. I'm fairly new to Mac but quite technology literate so not sure if I'm missing something. Any ideas? Thanks!

My Late-2012 iMac is running Catalina 10.15.7, so I agree with me too.

My Late-2013 iMac is running Mojave 10.14.6, so I agree with dialabrain.