why is OS X El Capitan unavailable

Have you previously downloaded El Capitan using your Apple ID?

Check the computer requirements shown in each link to see if your computer is capable of running that OS version.

How to upgrade to macOS Mojave - Apple Support

How to get macOS High Sierra   

Sierra download

You can try to download it at the link below. If it won't let you, then your only options are to upgrade to Sierra/High Sierra/Mojave or find an older computer where you can download it using your Apple ID.

El Capitan

Check to make sure your applications are compatible.

Application Compatibility

El Capitan 10.11 Compatibility information 

macOS Sierra 10.12: Compatible Apps

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

Your model must be able to run a later version of macOS. You need to download and install the latest version of macOS your model supports.

To install El Capitan means you must first install a prior version of macOS or already have a bootable El Capitan installer on a USB stick.