I want to download El capitan but it doesn't seem available. Apple says you can download it but it is not on their site?

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

OS X El Capitan - Technical Specifications

Safari will open a redirect and open the Apple Store where the download can commence

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal  operation of the OS and may even Corrupt the OS requiring a Reinstallation

Downloaded fine for me.

It should download a file called “InstallMacOSX.dmg”

Just tried it again, and it works fine. Click here-> OS X El Capitan Download

Just tried it again. If you have an antivirus or firewall make sure downloads are allowed. It may be blocking them.

What happens when you click the link for El Capitan in the support document you posted from?

Just tried it and it worked fine. It started to download. click here-> OS X El Capitan

Nothing happens when you download the link/file. Still doesn't give me 10.11