Mac OS X 10.6.8 App Store not working
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.