El Capitan bootable installer on Mac Mini - "A required download is missing"
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A lot of macOS installer apps downloaded before Oct 2019/Nov 2019 contain an outdated certification chain and thus are invalid.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.