If this message pops up: "Incomplete payment. The purchase was declined on the STC billing account. Please select a different payment method."

Please follow these steps:
1- Ensure that you have sufficient balance for the purchase, considering the value-added tax charged by Google and the store's country.
2- Clear the payment method information, re-enter it, and try to buy again.
3- If the process does not work, change the payment method.