If you select your supervisor's name and encounter an error message at the bottom of the page while trying to book a session, it likely means your supervisor isn't available at that specific date or time. You can try booking for alternative dates when your supervisor is free, or you can message them directly to coordinate a session at a time that works best for you.
Please note that even if a supervisor is not added to a session, they can still join, as they have visibility and access to all care providers' sessions.