You cancel a booked class yourself in the Workit app. Note: each sports location has its own cancellation period. You can cancel a booking up to a certain number of hours before the start; after that, cancelling in the app is no longer possible. The app shows until when you can cancel each booking.
Too late to cancel, but you really can't make it? Send the sports location a message yourself; sometimes they can still arrange something. That way you avoid being marked as a no-show.