Here are some potential reasons for Circleback not being able to join a meeting.
Host did not admit Circleback into the meeting.
Meeting platform's maximum waiting room time was exceeded.
Meeting is configured to not allow guests join or registration is required.
The password in the calendar event or in the entered meeting URL is incorrect.
When these errors occur, you can either try adding Circleback to the meeting via the URL to retry or capture the meeting with the desktop app.