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.
Disable Only authenticated users can join meetings in Zoom by following the instructions here.
Disable Registration Required for webinar in Zoom by following instructions here.
Select All users (including users not signed in with a Google account) from admin.workspace.com → Apps → Google Workspace → Google Meet → Meet safety settings → Domain by following the instructions here.
Enable Anyone with the meeting link can ask to join in Host controls for Google meet by following the instructions here.
The password in the calendar event or in the entered meeting URL is incorrect.
The meeting was set to not allow external participants to join.
To fix this you can mark "Anyone with the meeting link can ask to join" on the Google Calendar invite to have Circleback join the call, or the admin for the workspace can go to Apps → Google Workspace → Google Meet → Meet safety settings to turn it on globally.
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.