I Missed My Turkish Airlines Flight Due to App Error and Got No Help

A relative of mine, visiting Turkey from abroad for the first time, arrived in Istanbul at 12:45 on flight TK2459 from Çukurova, which departed at 11:00. However, we were not allowed to board the 14:10 TK9 Istanbul–Los Angeles flight on the grounds that we were late for check-in. As soon as online check-in opened, we tried multiple times via the app, but the system kept giving errors and we could not complete the process. Since we had purchased the TK2459 ticket weeks in advance, we had no option to change it or buy a new ticket. After being denied boarding, our request to transfer to the next flight or purchase a discounted ticket was also refused. Furthermore, when we applied for a refund for the missed flight, no assistance was provided. We paid approximately $2,600–$2,700 for two people, yet our grievance was not addressed and we were offered very high prices for new tickets. When we contacted Turkish Airlines customer service, we were only told to submit feedback, with no solution provided. Despite holding standard economy class tickets, we received no support. I request that our grievance be resolved by either issuing a new ticket or refunding the cost of the new ticket we are forced to purchase.