Turkish Airlines Refused Boarding Without Return Ticket

Turkish Airlines did not allow me to board the plane and is not providing a refund. On my Belgrade-Istanbul-Panama-Santiago flight, I boarded the plane in Belgrade without any issues. In Istanbul, I completed the check-in process without any problems, and at no point was I informed that I would not be allowed to board the plane without a return ticket. When I was about to board the plane in Istanbul, the staff at the gate did not allow me to board. Due to time constraints, I could not purchase a return ticket. One of the staff members told me that I could get a refund or change my ticket. Trusting the Turkish Airlines employees, I went to change my ticket and buy a return ticket. However, when I got to the ticket counter, I was told that my ticket was void and I could not get a refund or make changes. I was severely inconvenienced. When I contacted the call center, they were not helpful either. I had planned to travel to Argentina after Santiago to tour Latin America, and despite having an Argentina ticket, I was treated this way.