If you experience issues when purchasing a product with Store Credit, please make sure you have enough credit to cover the price of the product.
To check the amount store credit you have, click your name at the top right and select Bitrefill Account Credits. Keep in mind that you can combine your store Credit for payments (BTC, USD, EUR and Rewards), which you can read about here.
However, please note that it is not possible to combine your Store Credit with an external payment.