All players will need to have an active Apple iTunes or Google Play Account linked to their device in order to make purchases. These accounts will need a credit/debit card or game cards attached so there are funds available to pull from for in-app purchases.
If a purchase is being declined you may not have enough funds available on your account, or the card attached to the account is not set up to make online purchases. Feel free to reach out to your financial institution to make sure your credit/debit card can make online and in-game purchases. You can also reach out to Google Play or Apple about the status of a purchase.