If you need to perform a clean install, you can create a bootable installer using a USB drive (at least 12GB) and Terminal: Connect your USB drive and rename it to MyVolume .
Once you have the InstallOS.dmg , you cannot simply copy it to a drive. You must convert it into an .app installer first. How do I install MacOS Sierra DMG to Bootable USB? Download Mac Os Sierra 10.12 Dmg
: Your Mac should be running OS X El Capitan, Yosemite, Mavericks, Mountain Lion, or Lion to perform a direct upgrade. How to Use the DMG File : Use Safari to download the file to your Downloads folder. : Double-click the file to open it. : Double-click the file inside to install the macOS installer into your Applications : Open your Applications folder and double-click "Install macOS Sierra" to begin the upgrade process. Apple Support If you need to perform a clean install,