Hi there 桐立,
Have a look at the information below:
To qualify for an Apple Music student subscription, you must be a student studying a bachelor degree, post-graduate degree, or equivalent Higher Education course at a University, College (U.S. only) or Post-Secondary School (Canada only). Junior, technical colleges, and special courses are also eligible in Japan.
We see that you came to Apple Support Communities from this article: Get an Apple Music student subscription - Apple Support. Make sure you followed the steps in that support article to get an Apple Music student subscription.
You'll want to reach out to Apple Support directly for further assistance. They're best-equipped to take a closer look at the issue and provide some additional guidance. This link will help you get started: Apple Support
Take care.