This is a common error when installing El Capitan on older hardware. It happens because the installer’s digital certificate has expired.
This process does not install the OS; it extracts a file named "Install OS X El Capitan.app" into your /Applications folder. This 6.2GB app is the actual tool you need to upgrade or create a bootable USB. Step 3: The "Expired Certificate" Fix mac os x el capitan 10116 installer dmg download fixed
(Note: This link will launch the Mac App Store application automatically.) This is a common error when installing El