Turkish Airlines, Online Ticket Change Issue

I am unable to change my Turkish Airlines (THY) ticket online, despite having the right to a fare-free change. I contacted the call center, but the representative couldn't change it either, citing reasons like 'fare calculation needs to be done.' Why is this happening? I've changed tickets before without any fare difference. Is this how the company tries to make a profit? If necessary, I will appeal to international flight rights. My ticket allows for fare-free changes, so why can't I change it online? Is this being done deliberately? It seems like online and mobile changes are intentionally being blocked! I enter my PNR number and ticket number, the ticket appears, but I get an error when trying to make changes. I am experiencing the same issue on both the mobile app and the website. Do I have to call by phone? Even when I called, it wasn't changed!