There are several possibilities for this case:
1. Please make sure the card has a logo like VISA or Mastercard.
2. The balance is not enough to cover the price of the subscription or item.
3. The card was not activated.
4. You entered the wrong PIN.
5. The gift card was already redeemed to another company. Note: In some cases, the entire balance is redeemed onto your account for a previous purchase and you cannot use the balance for a separate company.
If you continue to have issues with your prepaid card or giftcard, you can contact Xsolla for further assistance: https://help.xsolla.com/i-cant-make-a-payment/gift-cards