Issues Activating Windows 10

My family recently got an iBuyPower pc from Costco, and we have it more or less setup, except we haven't been able to activate Windows 10 (we have the watermark in the lower right corner.) I found a product key through the Windows Command Prompt interface, but when I punched the key in, Windows said the key had already been used on another device. At this point I think I just need a new key. I've tried reaching out to IBuyPower's technical support, but I can't get through on the phone. I've sent an email, and I guess I'm hoping posting this will increase my chances of getting help from someone. I'd rather not spend the money on a new key, and I'd rather not go through the hassle of returning the pc, so hopefully I can get some help from someone.