In order to cancel a checked-in reservation, we should first Undo the check-in and then cancel it.
To Undo the check-in, please head to the Manage ( Details of the reservation) and navigate to the Status tab:
Click on Undo check-in and provide a reason for the action:
You will then be able to Cancel this reservation on the same page:
If an error mentioning the Editable history window pops up, click on the following article to extend it.