Unable to Change Turkish Airlines Flight Date Online Despite Choosing Flexible Option

I specifically purchased my Turkish Airlines Istanbul-Hannover flight for September 11th with the option to change the flight date without extra fees. Now, I need to move the flight to September 9th, but I am unable to do so either online or through the app. When I called the customer service center, they informed me that I need to pay a "₺594 service fee" to change my flight.

I don’t understand why you force customers to pay extra fees through the call center when none of your online platforms work properly. The call center representative said they could easily make the change from their end, but despite trying different computers, phones, tablets, and internet connections, I still can't change my flight. You can't leave customers in such a difficult situation.

I expect this issue to be resolved as soon as possible because I urgently need to change my flight date. I specifically chose a flexible ticket to avoid extra fees, so why should I be forced to pay because your system isn’t functioning? I'm urgently waiting for a solution to this problem.