Uber Eats Won't Let Me Use My Own Coupon and Customer Support Hasn't Fixed It

I have been trying to use a coupon that has been linked to my Uber Eats account for a long time, but every time I reach the payment step, I get a "payment error" and cannot complete my order. Even though the coupon has not expired and is clearly assigned to my account, the system simply will not let me use it.
I previously contacted Uber Eats customer service about this issue and was told it would be looked into, but time has passed and the coupon still has not been fixed — I keep running into the same error. I am trying to place orders through Uber Eats in Istanbul but am unable to take advantage of this promotion because of this ongoing technical issue.
I am requesting that the coupon currently linked to my account be made fully functional, or that a new coupon offering the same benefit be added to my account to make up for the inconvenience I have experienced.