@ZhaoweiZhang ...
You need to edit the discussion topic in your course and look to see if the "Graded" checkbox is selected or not. It would seem that your "M4: Discuss the Readings" topic is graded...just based on what I am seeing from your attached screenshot. But, it never hurts to check. You can follow the instructions in this Guide to edit a discussin topic:
How do I edit a discussion in a course? - Instructure Community - 1075 (canvaslms.com)
Go to your "Discussions" page, and then click on the topic name. Click on the "Edit" button, and then look to see if the "Graded" option has a checkmark next to it. If there isn't a checkmark, then you need to select that option and save your changes to the topic. Also, just double-check to make sure there aren't any other settings for that topic that would seem to indicate that you couldn't view the responses in the SpeedGrader.
I hope this helps a bit. Keep us posted here in the Community...thanks!