Before installing, ensure your Mac is compatible with Sierra (generally models from late 2009 or newer): : Late 2009 or newer.
macOS Sierra 10.12 only runs on these Macs: Download Mac Os Sierra 10.12 Dmg
For advanced users on macOS Catalina or later, you can try fetching the installer using this command: softwareupdate --fetch-full-installer --full-installer-version 10.12.6 . Hardware Compatibility Before installing, ensure your Mac is compatible with