SunExpress Airlines Website Glitch Prevented Me from Adding Seats

I tried to add selected seats to an existing flight, but there's an error on their webpage (I tried three different browsers) and their app, which asks for business tax details for an invoice. I don’t need an invoice, and even when I enter the information, it still won’t let me proceed to payment. The online web chat help simply said, "Go to the airport" and "Next time, add your seats at the time of booking." Absolutely terrible support desk.