After configuring your system open App Store.app and download OS X El Capitan. If you haven't purchased it previously use the answer in How to download OS X El Capitan from the App Store even if never downloaded before macOS Sierra was released or purchased is grayed out.
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I've been looking for El Capitan for weeks and today I finally managed to get it.
The Appstore no longer has OS X El Capitan available to download.
To solve this issue follow this link: http://bit.ly/ElCaptanOriginalOSX
How To Get Mac Os Dmg Without App Store Free
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Download Mac Os Sierra Dmg Without App Store
The Appstore only has updates but not the original OS. So If you did not download El Capitan before it was removed from the App Store then you will not be able to get it from the Appstore anymore. You can now only download it via alternative sources, Just make sure they are trusted sources though. Hence I recommended the link above.