softwareupdate --fetch-full-installer --full-installer-version 10.12.6
To download the DMG file, it is highly recommended to use official Apple sources to ensure the file is secure and untampered. Direct Official Download Mac Os Sierra 10.12.6 Dmg File Download
If you need to perform a clean installation, you can create a bootable USB drive (minimum 16GB recommended). Apple Support macOS Sierra (10.12.6) Download - Apple Support Community 24 Feb 2020 — Mac Os Sierra 10.12.6 Dmg File Download