I am not a mentor for MLS, but just looking at the error message it looks like an “IT” problem: notice that the DNS server is saying it can’t find the IP address for that coursera.org hostname. Is this the first time you are opening an assignment notebook in this course? If not and you have successfully opened other notebooks before, then the next question would be “is anything different about the current situation versus the previous cases that worked”? E.g. that you are using a different computer or you are on a different network (now at school or work but you were at home before)?
If everything is the same and this is not your first notebook, then the next theory is that something is damaged about that notebook. If that is the case, then try getting a clean copy. Here is the procedure to do that for DLS notebooks and I hope the same method will work for MLS as well.