softwareupdate --fetch-full-installer --full-installer-version 10.12
Because Apple makes the official download difficult, most users turn to tech blogs, "Software Library" sites, or the Internet Archive (Wayback Machine). Download Mac Os Sierra 10.12 Dmg
The DMG file does not contain the OS directly; it contains a package that installs the "Install macOS Sierra" app into your Applications folder. most users turn to tech blogs
This will "install" the installer app into your Applications folder. "Software Library" sites