Cancellation policies vary by accomodation. These specific rental terms which apply to your reservation are mentioned in your received confirmation or invoice.
In hotels on Schiermonnikoog 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.) on Schiermonnikoog, you often do owe a cancellation fee.
It's best to contact the accommodation owner directly for more information.