Open the mounted volume and double-click the package file.
Inside the mounted image, double-click the .pkg file (often named InstallOS.pkg ). download macos sierra 1012 dmg file install
Turn off your Wi-Fi network and disconnect any active Ethernet cables to stop your Mac from syncing with online network time servers. Open the mounted volume and double-click the package file
softwareupdate --fetch-full-installer --full-installer-version 10.12.6 download macos sierra 1012 dmg file install
Follow the prompts. This will install the OS yet; instead, it extracts the "Install macOS Sierra" application into your Applications folder. Step 4: Install macOS Sierra
Open the application ( Applications > Utilities > Terminal ).