Can't upgrade from High Sierra to Mojave or Catalina.

I've never had this issue before. I literally cannot upgrade to Mojave or Catalina. Neither show up in a search of the App Store. A google search turned up a site with a link to Mojave in the App Store. When I click to install it, I get the message "This copy of the install macOS Mojave application is damaged, and can't be used to install macOS."

With more research, I decided I would be better off with Catalina anyways. I can't find that in the app store at all. I did find an apple support downloads page with a bunch of security updates and combo updates for Catalina, but I'm not sure which is the one I need for a fresh install.

I'm on a 2018 MacBook Pro. What am I missing here? I can't go up to Big Sur just yet.