: This is the most common method. You can use a direct link to the macOS Catalina page in the App Store. Once downloaded, the file "Install macOS Catalina.app" will appear in your Applications folder.
This happens when the DMG was repackaged by a third party. In Terminal, run: hot download macos catalina 10157 dmg
If you already have an older version of Catalina (e.g., 10.15.0) and just need the 10.15.7 update file: : This is the most common method