Even without running Terminal commands, you can perform a simple integrity check:
This typically occurs when Apple's servers are overwhelmed or your connection is unstable. Solutions include:
macOS Catalina on the App Store
Once the App Store opens to the Catalina page, click the button.
To download the complete Catalina installer, copy and paste this command:
sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled Use code with caution.