Press the button while holding down the Option (Alt) key. Release the key when the Startup Manager screen appears. Select Install macOS Sierra and press Enter.
Fixing a corrupted operating system when the recovery partition fails. Official Method: Download macOS Sierra via Apple mac os sierra 1012dmg download free
: This link leads to the official Apple Support page where you can find the direct DMG download for Sierra 10.12. Press the button while holding down the Option (Alt) key