Other students have reported this error on different notebooks over the last week or so. Here’s another thread about this in Course 5 a couple of days ago. It looks like some kind of intermittent problem in the Coursera AWS backend. The only thing I can suggest is to wait and try again. In the other instances we’ve seen, it usually clears up within 4 to 6 hours. You can also try reporting this to the Coursera Help Center, but as you saw on that other thread students who reported that were told by Coursera to contact their course Forums. Even though it does legitimately appear to be a Coursera problem at least IMHO.
Of course it could also be a problem at your ISP or with the DNS servers on your network, but in that case you’d expect to see other problems with your web access in general. If DNS is not working, basically nothing works.