Step 1: Undo check-in
In order to cancel a checked-in reservation, you 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:
Step 2: Cancel the reservation
Then on the same page, find the field Cancel, select the reason, make sure you apply the cancellation fee if applicable, and cancel this reservation.
See the following article if an error mentioning the Editable history window pops up: