I had to reinstall with a Sierra bootable USB drive. Its really time consuming and might make you want to throw out your Mac but I'll post a couple forums that were really helpful. Excuse the incorrect terms as I'm not really a mac user.
Also, you have to have another working mac to do this.
Install Sierra into your applications folder from here:
Then double click the dmg file. After it opens double click the pkg file. then follow the steps presented. Don't install the Sierra installer onto the working mac.
Make sure you have a USB drive in the device then format the drive to Mac OS Extended.
You'll need to open the terminal and type: sudo su enter
Then the following commands: This first one solves a but with the creation media permissions.
sudo plutil -replace CFBundleShortVersionString -string "12.6.03" /Applications/Install\ macOS\ Sierra.app/Contents/Info.plist
sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
Refer to the following forums to complete the process. I believe you put the USB in the broken device and boot it. when it makes noise hold the option button. you should have the option to select the USB. once it boots then you'll need to be sure the internal drive is wiped. when it is and in the right format you should be able to install Sierra.
This took me forever to figure out but non of the other options would work for me so use this as a last resort. The following forums can help with issues you might have.