El Capitan bootable installer on Mac Mini - "A required download is missing" On a freshly created USB bootable disk for El Capitan (following these instructions which create a USB bootbale installer with sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app)

On a Mac Mini 2014 when trying to install El Capitan off the bootable USB

I am getting this message  "A required download is missing"

the machine is connected via ethernet to the upstream network



my hard drive is 500GB and reformatted Mac OS (Journaled)
You probably used an old macOS installer app to create your USB bootable disk.

A lot of macOS installer apps downloaded before Oct 2019/Nov 2019 contain an outdated certification chain and thus are invalid.

Simply download a new OS X El Capitan install.dmg (with a valid cert chain this time), open the dmg and open the InstallOS.pkg. The installer package installs the "Install OS X El Capitan.app" in the Applications folder.

The app's ressources can be used to create a new installer thumb drive with the well known command:

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app


Other installers with valid cert chains:

Sierra: How to upgrade to macOS Sierra – Download macOS Sierra (direct d/l).
High Sierra: How to upgrade to macOS High Sierra – Get macOS High Sierra (App Store link)
Mojave: How to upgrade to macOS Mojave – Get macOS Mojave (App Store link)

As of recently (October 2019) users have reported a certificate issue that may require the system datetime to be backdated via terminal in the installer.