HOW TO USE EXISTING $$ ON ITUNES FOR PURCHASES

Itunes purchases will use any available iTunes gift card credit or balance to pay before attempting to charge the associated payment method:

Read hear for more details:

How iTunes Store purchases are billed - Apple Support

If you have gift cards or content credits

If you use a gift card for a transaction, the amount is deducted at the time of your transaction.

When you make a purchase, content credits are used first, as long as the value of the item that you're buying is less than or equal to the value of the credits. Any gift cards are used next. If your gift card is less than the total cost of the purchase, the payment method that you have on file is billed for the remaining balance.

It will only ask for payment method if the purchase plus applicable taxes exceeds the amount of money in the account.

What are you trying to buy? How much does it cost, and how much credit do you have in your account?

Also please stop the all caps, it makes it hard to read.

There is no need to shout.

How items are paid for is automatic, you can't change it. To buy something click on its price button. If you are getting an error message or prompt when doing that ... ?

Try posting without shouting at us.

Does your balance cover the total price (including any applicable taxes) of what you are trying to buy ?

What is the exact text of the message that you are getting ? And you are currently in the country that you have as the country on your account ?