MacOS Catalina update

If you are using Mojave then you should find the Catalina upgrade in System Preferences> Software Update.

Also did you try the link I posted earlier, How to upgrade to macOS Catalina – Apple Support

go to Section 4 and click on Get macOS Catalina. Make sure you use Safari as your browser.

Well it is definitely there, just type macOS Catalina into the Search field, click on the link below,

How to upgrade to macOS Catalina – Apple Support

Use Safari as your browser.