Unable to make purchase with funds loaded to iTunes via Apple gift card

when trying to make a purchase it is unable to complete purchase

To keep this succinct, I believe the above is the key information. You say you loaded a gift card, but do not say that was the problem so I do not think we have to say anything addressing card redeeming issues.

Do you see a "Your purchase could not be completed" message?

A possible reason for seeing this message is Apple cannot find a functioning payment method to use in making this purchase. You may not have adequate balance, the kind of payment method required, or there may be problems with the method(s) 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.

If the information above does not track down 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.

You can use a separate account for iTunes purchases. In the support article: If you subscribe to Apple One and use a different Apple ID for iCloud - Apple Support Apple alludes to using a different Apple ID for media subscriptions and purchases.

trying to make purchase with funds loaded to iTunes via Apple gift card

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.

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.

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If your payment method is declined in the App Store or iTunes Store, see -> 

If your payment method is declined in the App Store or iTunes Store - Apple Support

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How to complete your purchase if a payment failed or was declined

1. On your iPhone or iPad, open the Settings app.
2. Tap your name.
3. Tap Payment & Shipping.
4. Add a different payment method and remove the old one. 
5. Try to complete your purchase.

If you still can't make a purchase

• You might have an unpaid order. Learn how to pay an unpaid order.
• Contact your financial institution. 
  • You might need to ask your financial institution to allow international purchases on your debit or credit card.
  • If you want to know why a debit card, credit card, or other payment method was declined, speak with your financial institution. Apple doesn't have information about why the payment method was declined.
• In some countries and regions, your payment method might be declined because of changes to local law. Learn about billing and purchases in Europe or what to do if your card is declined in India.
• Still need help? Contact Apple Support.

See the following Apple articles:

Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.

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)

⤵️

• 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 need to change or update your Apple ID payment method

If you need to change or update your Apple ID payment method - Apple Support

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

• If you want to change your card, you need to add first the new card, then remove the old card.
• or if you use your Apple Account balance, you need to set your Apple Account balance as your default payment.

Add money to your Apple Account balance -> Add money to your Apple Account balance - Apple Support

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 have followed all the steps and instructions and if you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.

By the way, you not need to set your Apple Account balance as your default payment. It is always your default payment method and you could't change that even if you wanted to.