changing payment details after gift card expiry

Hey,
So I subscribed to an App for a month with a Gift Card. The Gift Card has expired now and I'm being asked to clear the payment. I can't do so until I change the payment method but the system doesn't allow me to do that by not accepting the new payment method. I can't even use Whatsapp until I update this to be able to update the software. Can you please assist me in fixing this problem.
Thanks