The best solution is a long one but the mod bot deleted my post. basically you need to make a bootable Sierra USB with a working Mac and install it but the long and short of it is that you'll need to use the terminal on another mac and install to the boot drive through commands. the issue is that there is a bug with the creation media tool for Sierra that makes the command unrecognized. It took me a week of research to get it to work. Probably mostly because there was only one forum that said anything bout the bug. and because I'm not really a mac guy. Good luck. If I can get the mods to let me post I'll send you the sites I found helpful.
Adk