when upgrading from macOS Mojave to Catalina, it says 'This update requires macOS version 10.15.'

Run this command in the Terminal then reopen System Preferences > Software Update.

sudo softwareupdate --reset-ignored

You will need the full Catalina installer from the System Preferences : Software Update panel, not an incremental update found on the Apple downloads site.

Finally a solution! Thank you. I've had this issues with my 2015 12" macBook since Catalina was released. Of course, Apple offered no help. Thanks again and Merry Christmas.

You're welcome. Same to you.