Hi @XIANXUE,
You may not realize it, but you posted your question to the global Instructure Community, generally comprised of Canvas and other product users around the world. Unfortunately, users here don't have access to any of your school or account information, so we won't really be able to provide an exact answer to this. I would recommend reaching out to your professor, perhaps via email or phone if available, to confirm that your'e on the course roster and that the course has been made available to students. if you don't have info for your professor, the next best place to call would be your specific institution's Canvas or IT help line, as they'd also likely have access to enough info to help you out.
I hope this helps guide you to the best place to get help for your issue.
-Chris