Upgrading my iMac OS from Mavericks 10.9.5 to El Capitan 10.11

I have an iMac (late 2013) 21.5" running Mavericks 10.9.5. I want to upgrade to El Capitan 10.11.

Cant find it anywhere that will upgrade from 10.9.5. About three months ago I was easily able to upgrade

an older iMac 24" mid 2007 running Mavericks 10.9.5 to El Capitan. What gives???