Student cannot join the team in the classroom

The error information is: “You are forbidden from performing this action on”.
I created a classroom and a group assignment. I give the invite link of the assignment to the student. Students come into the group assignment. But students encounter a error when they create a team or join a team. The error information is showing in the beginning.
Later I look for the reason, and I found maybe because the student has not accepted the assignment. But the student and I cannot find where to accept the assignment. Can anyone give me some information or some resolution?

@wpxu08 are they trying to add a team on the Classroom front-end or on the GitHub organisation page?

They just clicked the invite link and go on…
Some student can join and some cannot.
I am very fused about this situation.