@Yinan_Wu Try changing your browser. Also can you confirm if your network connectivity is good. I have faced such issue when I had network related issue and when I tried after sometime(sometimes I tried after 12 hrs), it worked.
if issue persist, then in the coursera, go to the learner help center, raise ticket, mention that you have already tried clearing the browser and cache history, and yet facing the problem with the screenshot you shared with us. they should be able to resolve the issue, if there is server issue from coursera side.