Can not download el Capitan
If neither of the "direct" App Store links (1. El Capitan 2. El Capitan) as described in other answers like this one work, you can try to trick App Store - as elaborated here: Install OS X El Capitan on Mac Direct Download.

Here a local web server with all necessary files mimics the remote but stubborn Apple server.


Open a browser and download the following files:


OS X El Capitan .pfpkg file (direct link)
OS X El Capitan .pkg file (direct link)

Open Terminal.app and enter 

sudo nano /etc/hosts

add the following line at the end of the file: 127.0.0.1  osxapps.itunes.apple.com
Hit ctrlO to write the file to disk and ctrlX to exit nano
Enter sudo killall -HUP mDNSResponder to flush the DNS cache.
Enter cd Desktop to change your working directory to your desktop
Create a folder mkdir osxapps_local and cd to it: cd osxapps_local
Create to paths with:

sudo mkdir -p ./apple-assets-us-std-000001/Purple60/v4/45/df/86/45df865d-d24d-01d6-2b2c-c003317293ac/
sudo mkdir -p ./apple-assets-us-std-000001/Purple20/v4/dc/94/05/dc940501-f06f-2a91-555e-3dc272653af5/

move the downloaded files to the respective folders:

sudo mv ~/Downloads/signed.dcr.6112397842917719871.pfpkg ~/Desktop/osxapps_local/apple-assets-us-std-000001/Purple60/v4/45/df/86/45df865d-d24d-01d6-2b2c-c003317293a
sudo mv ~/Downloads/izt4803713449411067066.pkg  ~/Desktop/osxapps_local/apple-assets-us-std-000001/Purple20/v4/dc/94/05/dc940501-f06f-2a91-555e-3dc272653af5

Start a small web server:

sudo python -m SimpleHTTPServer 80

Now you can check if your small web server works by entering 

http://osxapps.itunes.apple.com/apple-assets-us-std-000001/Purple20/v4/dc/94/05/dc940501-f06f-2a91-555e-3dc272653af5/izt4803713449411067066.pkg 


in the address field of your browser. The file should be "downloaded" with a high download speed. Delete the downloaded file afterwards.
Launch App Store.app, go to the El Capitan page or your Purchased tab and download OS X El Capitan from there.
The OS X El Capitan Installer.app should be "downloaded", compiled in /Applications/ and launched. Quit the OS X Installer for now. Quit App Store.app.
Quit the http server by entering ctrlC in Terminal.app
Revert the changes of your hosts file:

sudo nano /etc/hosts


and remove the line 127.0.0.1  osxapps.itunes.apple.com.
Hit ctrlO to write the file to disk and ctrlX to exit nano
Quit Terminal.app
Open /Application/Install OS X El Capitan.app and install El Capitan