Error 403 trying to reinstall OS X El Capitan on iMac (late 2012)

I have a late 2012 iMac that I purchased used. I was unable to login to it without the seller's password so I erased the disk and am now trying to reinstall OS X. Upon install I receive an Error 403. I tried resetting the date/time which I had seen as a resolution in other forums but it did not work for me. I am connected to Wi-Fi but don't have access to connect ethernet. Am I looking at professional repair?