If you haven't paid for the reservation yet, there are no cancellation costs.
After payment, you may cancel up to the point when your trip starts. Do you have a Plus or Pro subscription? Then you may cancel without penalty.
Do you not have a subscription? If you cancel your reservation at least 24 hours in advance, then you will not pay any cancellation charges. If you cancel your reservation 24 hours in advance or less, then you will pay 10 euros in cancellation charges for each day you had booked.