How do I know if I have an unpaid balance

See -> View your purchase history for the App Store, iTunes Store, and other Apple media services

View your purchase history for the App Store, iTunes Store, and other Apple media services - Apple Support

• Check all your current downloads.

If you can't make a purchase, check also -> If your payment method has been declined in the App Store or iTunes Store -> If your payment method has been declined in the App Store or iTunes Store – Apple Support (UK)

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How to complete your purchase if a payment has failed or been 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 may have an unpaid order. Find out how to pay an unpaid order.
• Contact your financial institution. 
  • You may 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 has been declined, speak to your financial institution. Apple doesn't have information about why the payment method was declined.
• In some countries and regions, your payment method may be declined because of changes to local law. Find out about billing and purchases in Europe or what to do if your card is declined in India.
• Still need help? Contact Apple Support.

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) 

Read this article about how to check your Apple ID balance.--> Check your Apple Account balance - Apple Support

If it says you have a negative balance then you owe money. Read the links below about various ways to pay for items in the iTunes and App Stores.

- Add a payment method to your account. See this link about payment methods that you can use with your Apple ID --> Payment methods that you can use with your Apple ID - Apple Support then see this link about how to add a payment method to your Apple ID --> Add a payment method to your Apple ID - Apple Support

- Add money to your Apple ID balance - "Use a payment method, such as a credit or debit card, to add funds to your Apple ID balance." This takes the previous payment method one step farther by optionally using it to irreversibly add funds to an Apple ID account balance instead of having the card charged each time.

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

- 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