Why can't I add interactive webinar user in the sandbox?

Hi there, I'm jiwon

I tried to add an interactive webinar user in the sandbox, but it failed.
When I add a user, the screen stops loaded. When I refresh the screen, it returns to the same screen as before I added a user.
(I attached a screen capture, please refer to it.)

Is it impossible to add an interactive webinar user in the sandbox?

I look forward to hearing from you soon.
Thank you.