Unable to use my redeemed Itunes credit! Keeps saying contact apple support whenever I wanna make a purchase.

A likely reason for seeing this message is Apple cannot find a functioning payment method to use in making this purchase. You may not have the kind required or there may be problems with the ones you do have.

Use the document Add a payment method to your Apple ID - Apple Support to review your payment methods such as credit or debit cards. Problem methods will usually have a notice in red lettering below them. The article: If your payment method is declined in the App Store or iTunes Store - Apple Support has some information about declined payment methods.

If using Apple Account balance to pay for an item, ensure there is adequate balance to pay for the item, including tax if applicable. Read this article about how to check your Apple ID balance.--> Check your Apple Account balance - Apple Support Some purchases may require the presence of another payment method such as a credit card even if the final payment will be made by account balance.

Providing a comprehensive listing of all possible issues and their solutions would be excessive in a single reply. If the information above does not indicate the nature of the problem then you may find the fastest route to resolution would be to accept the invitation to contact Apple for assistance with this.

To contact iTunes Support, do this:

1. Click on this link --> Choose your country or region - Official Apple Support

2. Select your country

3. Select: Billing & Subscriptions

4. Select: Subscriptions & Purchases

5. Select: Update payment method

6. Select a chat session or a phone call.

Incidentally, you do not need to set your account balance as the default payment method. It is automatically used as that and you cannot change it even if you wanted to.

See -> How to redeem your Apple Gift Card or App Store & iTunes gift card

How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)

When redeeming a gift card, the amount will be credited to your (1) Apple Account balance and (2) you need to set your Apple Account balance as your default payment.

(1)

See -> Check your Apple Account balance

Check your Apple Account balance – Apple Support (UK)

(2)

See -> Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

If you already use your Apple Account balance, the Apple Gift Card or App Store & iTunes gift card is automatically added to your Apple Account balance.

If you update your payment method or add funds to your Apple Account balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.

See also -> What you can buy with your Apple Gift Card or Apple Account balance

What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)

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• You can't use your Apple Account balance to send gifts or gift cards.
• If you use Family Sharing, your family members can't spend your Apple Account balance. Funds that you add to your Apple Account balance are only available to you. Find out more about how purchases are billed to your Apple Account balance.
• Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.

If you have a valid payment method on file, instead of buying or order an App Store & iTunes gift card, you can Add money to your Apple Account balance anytime, no gift card needed, see -> Apple Gift Card - Apple (UK)

if you still need help, get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

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See also this Apple article:  If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

Beware of -> About Gift Card Scams -> About Gift Card Scams - Official Apple Support

I'm having the same issue, repeatedly, over and over. I have put gift cards into my Apple ID account, so there's plenty of money to cover my in-app purchases (please do not reply back with - make sure you have enough money to cover the purchase...). I'll usually be able to make a few purchases and then I get the error telling me to contact Apple Support. I sit on a chat for a few minutes and they tell me 'Apple works to keep purchases safe blah blah blah' then they 'escalate the issue' and ask me to try the purchase again. Then it works. For a while. It seems that Apple's algorithm isn't happy if you put gift cards into your Apple ID account as this must be how some fraudsters do their dirty work. So, for the rest of us, we're stuck having to reach out to Apple repeatedly.