If you want to cancel the appointment via the customer's link and the appointment no longer works, check your settings to see if the default cancellation time has been undercut. Click on your profile picture in your profile and go to "Profile".
Switch to the "Booking Widget" settings. Here you can define the cancellation time in the lower area. (Standard setting for cancellation until)
You can also change or delete appointments in the online calendar itself. For this it does not need the customer link.
If it is a group appointment, you have to edit the corresponding group appointment. Go to the group event page and click on "Edit" at the corresponding group event. Under "Edit group appointment" you can set the logout time.
This way it is possible that a customer cannot delete his appointment because the cancellation time has fallen short.