According to the KLM cancellation policy, passengers can cancel their reservation and request a refund after meeting the following requirements:

Customers must cancel flights to/from the United States within 24 hours of making a reservation on the airline's site. Conditions apply to all ticket types and travel classes.
You can claim a full refund or travel credit for flights canceled by KLM Airlines. It is also your choice whether or not to take an alternate flight on the same date.
If the airline reschedules a flight that is no longer required, you will receive a full refund.
Eligible passengers may request a full refund by completing the online refund request form on the airline's official site. You must have a fully flexible KLM reservation in any travel class.
There are unavoidable circumstances, such as death or serious injury before the departure date. The airline will waive the flight cancellation fee and initiate a full refund.
If your visa is denied, you can apply for cancellation risk-free and get a full refund.
In accordance with KLM's refund policy, passengers will be compensated for international flight cancellations delayed by more than 3 hours.
Customers should verify that they have purchased tickets from third parties and contact them for cancellation or refund requests.