The most reliable source to download the macOS El Capitan 10.11.6 installer DMG file is directly from Apple's Support website. However, Apple usually provides the latest version of macOS through the Mac App Store.
The safest way to get the installer is directly from Apple Support , which provides a verified "fixed" DMG file that includes the necessary certificates to prevent "expired" installation errors. mac os x el capitan 10116 installer dmg download fixed
A fixed DMG is useless if you cannot make a bootable installer. Here is the correct command for El Capitan 10.11.6: The most reliable source to download the macOS El Capitan 10