How to reinstall El Capitan "Unavailable Error"

You can make a bootable USB stick to install using this free program which will do all the work for you.

Bootable USB Flash Drive – Diskmaker X oruse Apple’s method Create a bootable installer for OS

If you purchased the computer from someone else who installed El Capitan using their Apple ID you cannot reinstall it, you must sign into the Mac App Store using your ID and download the latest Mac OS that is available for your machine. If your Mac is a 2011 or later you can use Internet Recovery (command-option-r) to erase the hard drive and reinstall the original OS then upgrade to High Sierra from there.

If you insist on using El Capitan you will need to sign into the App Store on a Mac that cannot run High Sierra and download EL Cap, then make a bootable usb installer to use on your MBP.

Open the App Store and click the download link for High Sierra.

Is there any help in this lnk (under the heading 'Get El Capitan from the App Store':

How to download OS X El Capitan - Apple Support

If your ID never downloaded it you can't download El Capitan.

Your machine probably supports High Sierra and that's the OS you should use.