I recently reinstalled Mac OS X on my iMac to Mac OS X 10.6.3 then updated to Mac OS X 10.6.8 and now I am trying to update to OS X El Capitan but when I go to the App Store to download, I click the "GET" button and nothing happens. Nothing also happens when I try to do this with any other apps.
There is a version of El Capitan specifically being kept available for any Mac that needs the stepping stone to Sierra, or that can run El Capitan but not Sierra.
It's on the App Store at https://itunes.apple.com/app/os-x-el-capitan/id1147835434?mt=12
You cannot use it to revert from Sierra to El Capitan, for that you need to already have El Capitan in your purchase history.
As far as I'm aware, but cannot test, you cannot use it to upgrade to Sierra if your machine could do it without the stepping stone.