System Error on Turkish Airlines Prevented Check-in and Caused me Unboarding

My brother tried to check-in for the flight from Istanbul to Dalaman on 18.09.2023 at 12:05 a day before, both via mobile and web. Due to issues with your system, it didn't allow him to check-in. We called customer service, and even your call center representative couldn't complete the check-in and advised us to do it at the airport. When we insisted if there would be any issues, they assured us there wouldn't be. Today, my brother went to the airport and tried to check-in at the kiosk but failed. He spoke to an attendant who directed him to the domestic flights queue. After a long wait, they told him that check-in was closed when his turn came. Since it was within 45 minutes of the flight, they directed him to a supervisor. The supervisor said they couldn't do anything after the 45-minute mark and didn't assist further. Due to your system, my brother was stranded at the airport. How can this be acceptable? And you claim to be an internationally recognized corporate company. How can you leave a passenger stranded saying you can't do anything? What kind of service is this? Additionally, why are we being penalized for your system's fault? I will complain about you everywhere and will never buy a ticket from THY again.