Describe a deviation from the contract with its date location and specific service. Identify the booking clearly and attach the key evidence. A factual message with a clear request for assistance is more useful than a general complaint without reference to the agreed services.
If you booked through an intermediary messages requests or complaints about the performance of the trip can generally also be sent to that intermediary. The intermediary must forward them to the organiser. To assess dates and limitation keep a record of when each message arrived.
Do not copy a deadline or demand from a template without checking the contract and legal basis. Preserve sending confirmation delivery notices and replies. For multilingual communication state the key point in a language that all relevant parties can understand.