Download Macos Catalina 10.15.7 'link' Full Installer -
sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyUSB (Replace MyUSB with your USB drive name.)
sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyUSB (Replace MyUSB with your USB drive name.)