Turkish Airlines Charged Me Excessive Additional Tax for Business Upgrade

THY charged a high amount of additional tax for a business upgrade.
I upgraded my THY Lahore-Istanbul flight through the THY app. I was given two options: 25,000 miles or ₺9663. I opted to pay ₺9663 for the Business upgrade. However, at the airport during check-in, I was charged an additional 146,000 Pakistani Rupees (approximately ₺16000) for unpaid taxes. I refused but had to pay because they wouldn't let me board the plane.

There was no warning or mention of additional taxes when I made the upgrade through the app. THY is misleading customers and not providing accurate information.
Despite repeatedly providing feedback to THY, I only received standard responses stating that the process was in accordance with regulations, without any proper explanation.
How can you charge an additional ₺16000 in taxes without any warning for an upgrade that cost ₺9663? I can't understand the logic behind this.
I hope THY corrects their mistake and refunds my money, otherwise, I will take legal action.