If you are on a Mac and have the installer in your Applications folder, use the createinstallmedia command: OS X El Capitan | Specs, reviews and EoL info - InvGate
If your Mac previously "purchased" El Capitan, it may still appear in your App Store history. Mac Os X 10.11 El Capitan Iso Download
Apple provides the macOS El Capitan 10.11 installer directly for users with compatible older hardware. If you are on a Mac and have