macOS Catalina software update isn't showing as a software update for my Mac, why?

I have macOS High Sierra and macOS Catalina software update isn't showing as a software update for my Mac. I've checked pending downloads, checked my computer for software updates, but not sure what I can do get the new software. I need the update to have Boot Camp run properly in order to install Windows on my mac. Any ideas on how to get the update when it's not showing in my App store?