Go to Software Update, click on the text area and press Command+R to force a refresh. That did it for me.

But, the download speed is very slow (it says "About 2 days remaining" for me). So, it doesn't matter if you can't see the update..

I'm downloading via Terminal now:

% softwareupdate --fetch-full-installer

More info:

https://osxdaily.com/2020/04/13/how-download-full-macos-inst...

Might depend on the OS version; in the past App Store had all the “big” updates and System Preferences only handled the minor ones. Now it handles all of them.

On Catalina, you can find it in the App Store (https://apps.apple.com/us/app/macos-big-sur/id1526878132?mt=...) but when you click 'Get' it moves over to Software Update. I got the download started after a few tries but it is crawwwllling.

You can force a check in terminal with

    softwareupdate -l