This is a known issue where appointment slot information becomes stuck and prevents the appointment from updating correctly.
Cause
The appointment still contains slot data that needs to be removed before the system can update the appointment status.
Resolution
A data fix is required to:
Remove the slot information.
Update the appointment status to Cancelled.
Because this fix must be completed by the Support Team, please type talk to a person in the Digital Assistant. You will then be connected to a live support agent who can complete the fix for you.