Snow Leopard, App Store unable to download anything
Years late, but to provide an answer...

You can only download the current OS X/macOS from the regular apps or search page.

If you require an older OS, then you must use your Purchase History &amp; download it from there.



Photoshopped to put these all next to each other

El Capitan is slightly different, as it has been specially preserved by Apple due to Sierra not being compatible with older machines.

If you need El Capitan, it can be downloaded from the App Store - though it's difficult to actually search for.
See Apple KB: Upgrade to OS X El Capitan which gives links that will open the correct location in the App Store app.

...or try this direct link