Cancellation policies vary by accomodation. These specific rental terms which apply to your reservation are mentioned in your received confirmation or invoice.
In hotels in Knokke-Heist you can usually cancel or change your reservation 24 hours prior to your arrival. You can read your 'Hotel Policies' and see if a cancellation fee will be charged.
Keep in mind that in other accommodations (vacation bungalows, apartments, B&B, chalets, etc.) in Knokke-Heist, you often do owe a cancellation fee.
It's best to contact the accommodation owner directly for more information.