Unfortunately when I try to download it from the App store, it tells me that it won't work on my computer, which is ridiculous. El Capitan came out only a few months after I bought this computer. I think they see that my computer is compatible with High Sierra and so they want to force me to download that instead. No thank you.
Try calling AppleCare. (With their new "We'll call you" procedure it's almost painless.)
