Prohibitory symbol on MacBook? I’m getting the prohibitory symbol when starting up my mac. I can get into recovery mode. Neither disk utility or reinstalling macOS Sierra are working in recovery mode. They both fail. It just says unable to unmount volume for repair.
﻿﻿﻿For some reason, your mac cannot find the startup partition. You have tried disk utility.  You can try resetting the machine. 

Sometimes if volumes don't appear in Startup Manager (what you get when you hold down the Option key at startup), you need to reset the Mac's PRAM, NVRAM, and Open Firmware. Shut down the Mac, then power it up, and before the screen lights up, quickly hold down the Command, Option, P, and R keys, until the Mac has chimed twice more after the powerup chime. 
https://support.apple.com/en-us/HT204063

Your next step is to take it in for repair or replace the drive.   The simplest solution would be to buy an external drive.  I like OWC external drives.