Airlines allow passengers to cancel their reservations. However, it depends on the airfare. For example, some tickets are sold at discounted or non-refundable fares. However, in such cases, if the passenger is unable to travel due to force majeure, the airline will issue a refund if proof is provided. then according to Thai Airways Cancellation Policy ypu have to provide that. https://www.cancelation-policy.com/thai-airways-cancellation-policy