Upgrading to MacOS from 10.6.8 MacBook Pro
Are you using a 32-bit machine?

Unfortunately, 32-bit systems cannot run any OS after Snow Leopard (10.6). You have a few options:


Keep using Snow Leopard (be extremely careful about security, as I don't think it's still receiving updates)
Buy a newer Mac (possibly used)
Install Ubuntu and use it instead of OS X


If your machine is 64-bit, then I don't know what the problem is.