Some providers collect cancellation fees if a client violates the cancellation policy. This policy can be found on the provider's business profile by clicking on Details, scrolling to the bottom of the page and selecting Payment & Cancellation Policy. This policy is also displayed at the moment of making a booking and can be found in the booking confirmation email.
If you adhered to the cancellation policy but still incurred a charge, you should contact the provider directly. You can find their contact details on their Booksy business profile.