Greetings @sukiekeita ,
Thank you for posting on the Instructure Community!
canvas.instructure.com, which is the domain your student registered with, is associated with the general free canvas account that instructors can sign up for and create a free course and add their students to it. In one of your images, you appear to be on the free for teacher instance but in the other you're in your organization's instance. Please ensure that your student is actually registered with an account for your organization's instance instead of free for teacher, as there is no connection between them, which is why your link isn't valid because she's using the link when in free for teacher but it's actually from your organization's environment.
Thanks, reach out with any additional questions or concerns!
Noah