sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ macOS\ Sierra.app Use code with caution.

This method only works if your current hardware is technically compatible with Sierra Apple Support 3. Minimum System Requirements Before installing, ensure your hardware meets the technical specifications Apple Support macOS Sierra - Technical Specifications - Apple Support

Downloading the file is only half the battle. The .dmg contains a package that places the installer into your Applications folder. This is a common point of confusion—remember, running the .pkg doesn't install the OS; it just "unpacks" the installer app so you can use it.

Running OS X 10.7.5 Lion or later to upgrade directly. Where to Download macOS 10.12 Sierra Safely