The link I have provided in my previous post will take you to the 10.15.7 Catalina full installer download in the Mac App Store. That is the only Catalina installer available.
If that download fails, please check your Applications folder for a previous download which may have placed the "Install macOS Catalina" app in that folder. If you find that or another "Install macOS..." app in the Applications folder, drag it (them) to the trash, empty the trash and try the download again.
If you have found Catalina installers at another site, Apple.com/downloads, you have found update installers that may only be applied to earlier versions of Catalina already installed. They are not full installers.