The U.S. Department of Transportation has enacted a law that airlines will not charge a cancellation fee if you cancel a ticket scheduled within 24 hours of booking. Singapore Airlines is subject to a Singapore airlines cancellation policy 24 hrs. You may cancel your ticket for free within 24 hours of booking as you are not sure about your trip. After 24 hours, you will have to pay a cancellation fee to get a refund. If you book a non-refundable ticket, only government tax will be refunded. The 24-hour policy is implemented for both non-refundable and non-refundable tickets.

https://www.cancellation-policy.com/singapore-airlines-cancellation-policy