You can most certainly cancel your reservation via the APP or online after you have checked in. I have done it many times. I usually only make a reservation in advance when the cancellation deadline is 11:59 pm the night before or on the day of the stay. Even while checked-in, cancelling a stay generates a cancellation number, moves the stay into the cancelled stays list and emails you a confirmation of the cancellation. No need to call CS.

If you are past the cancellation window, the message feature is your friend. On several occasions I was unable to get to the property due to circumstances beyond my control. In one case, they cancelled my stay without a fee, in the other case, they shortened my stay by one night arriving a day later, again without a fee. Don't ever no-show.

James