Activate Windows on bottom right corner. Build 9600

There is an "Activate Windows Go to PC settings to activate Windows" and Windows 8.1 Pro Build 9600 stuck on the bottom right of my screen. I bought this computer originally off craigslist and this started to happen. I don't have a product key because the guy i bought it from didn't give me it. I need this solved before 7/29 because that's the expiration date for the free windows 10, and they won't let me update without this fixed. Please help!