Cant spend my account balance

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, (1) the amount will be credited to your Apple Account balance and (2) then 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.

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)

and ⤵️

• 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 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:  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

Re: "Cant spend my account balance : I bought 15$ itunes gift card, but when i want to spend that,

i’ve recieved an error message (purchase could not be completed) why is that?"

Gift Cards need to be redeemed (in the country or region where they were purchased) in order to use the funds:

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

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

See the article below for what payment methods Apple can accept in your country or region:

Payment methods that you can use with your Apple ID - Apple Support

If you are using funds from a Gift Card, some purchases require a credit card on file, even if your Gift Card balance is used for the payment.

See: What you can buy with your Apple Gift Card or Apple Account balance - Apple Support