You can download the macOS Sierra 10.12 DMG installer directly through official Apple support pages
Conclusion
If the App Store link fails, open Terminal (Applications > Utilities) and run: Download Mac Os Sierra 10.12 Dmg
Ensure your Mac meets these minimum requirements before attempting the installation: : At least 2GB of RAM. : At least 8.8GB of available disk space. Supported Models : Late 2009 or newer. MacBook Pro : Mid 2010 or newer. MacBook Air : Late 2010 or newer. : Mid 2010 or newer. : Late 2009 or newer. : Mid 2010 or newer. Creating a Bootable USB You can download the macOS Sierra 10
Direct Download: Access the official macOS Sierra 10.12 DMG directly through a web browser (Safari is recommended for compatibility). MacBook Pro : Mid 2010 or newer