Upgrading to macOS Sierra requires OS X 10.7.5 or later
As part of Apple’s Upgrade to macOS Sierra documentation, there’s been a change in the system requirements for macOS Sierra as opposed to OS X El Capitan.
For OS X El Capitan, the earliest OS you can upgrade from is Mac OS X Snow Leopard 10.6.8.
For macOS Sierra, the earliest OS you can upgrade from is OS X Lion 10.7.5.
If you’re upgrading from 10.6.8, Apple’s guidance is to upgrade first to El Capitan and then to Sierra.