Anonymous,
Thank you for your question! That is definitely a strange error, and I am wondering if it has to do with your university email. Have you attempted to use your personal Microsoft account? Since this is a sandbox environment, you will receive no charges to your Microsoft account so I don't think there is any reason this wouldn't work.
Try heading to aka.ms/az900/sandbox and signing in with a personal Microsoft email and attempt to launch "Exercise - Explore the Learn sandbox - Training. If that works, I would recommend submitting a ticket with Microsoft to ask why your student account won't work with the sandbox environment.
Thank you,
Joseph