If the App Store is not working, you can download the installer directly from Apple. Download macOS Catalina 10.15.7 via Apple Support 5. Troubleshooting: "The Update Cannot Be Installed"
That is the only official, supported way to get Catalina from the Mac App Store today. macos catalina 1015 19a583 mac app store dow
Then reboot and reopen App Store.
Wait for the download to hit 100%. The file Install macOS Catalina.app will appear in your folder. If the App Store is not working, you
: Catalina requires roughly 20 GB of free space for a smooth download and installation. macos catalina 1015 19a583 mac app store dow