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
Whether or not you've downloaded it in the past, you can now download OS X El Capitan 10.11.6 (6.21 GB) via this link which will open first in the Browser, then in the App Store.


  OS X El Capitan
  
  This version of OS X El Capitan is for users running OS X Snow Leopard
  who would like to upgrade to macOS Sierra. If you are running Snow
  Leopard and would like to upgrade to Sierra, you need to install El
  Capitan first. This version of El Capitan can also be installed on Mac
  computers that are not comparable with Sierra.











Note: If doing this to downgrade from macOS Sierra.

In order to use the Install OS X El Capitan.app from macOS Sierra you need to create a USB Installer and boot the Mac with it.  You'll also need to erase the Macintosh HD and perform a clean install.  Make sure you have proper User Data Backups from which to restore your Data before erasing the Macintosh HD.

Have a look at Create a bootable installer for OS X. Excerpt show below:


  Use the 'createinstallmedia' command in Terminal
  
  
  Download the OS X installer from the Mac App Store. Quit the installer if it opens automatically after downloading. The installer
  will be in your Applications folder.
  Mount your USB flash drive or other volume. You could also use a secondary internal partition.
  Open the Terminal app, which is in the Utilities folder of your Applications folder.
  Use the createinstallmedia command in Terminal to create the bootable installer. Examples of this command are in the next section.
  For detailed usage instructions, make sure that the appropriate
  Install OS X app is in your Applications folder, then enter one of the
  following paths in Terminal:
  
  
  Path for El Capitan:

/Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia

  
  Examples
  
  This is the basic syntax of the command. Replace volumepath with
  the path to your USB flash drive or other volume, and replace
  installerpath with the path to the Install OS X app. 

createinstallmedia --volume volumepath --applicationpath installerpath

  
  The following examples assume that the OS X installer is in your
  Applications folder and the name of your USB flash drive or other
  volume is MyVolume:
  
  Example for El Capitan:

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