Turkish Airlines Denied Me to Board So I Experienced Significant Inconvenience

Today, I arrived at the airport for my TK7053 Elazig-Ankara flight. Yesterday, on 07.12.2023, I completed my check-in with my PNR number RIVZTH. My seat number was assigned as 17C by the system. When I tried to get my boarding pass at the airport, I was told that I was not visible in the system, had not checked in, and would not be allowed to board the flight. I insisted that I had completed the check-in process. Despite this, I was not allowed to board and was not given a boarding pass. I experienced significant inconvenience. I had to purchase a new ticket for the 05:40 AM flight. Since I had already checked in, I knew my seat number. Additionally, while I was being denied boarding, passengers were still being allowed onto the plane. When I asked if there were any available seats, the staff responded, 'There are no seats available.' I would like to understand the reason why my seat was sold to someone else despite having checked in. This is the first time I have encountered such a situation in my life, and I am filing a complaint.