I can’t change the region in the App Store to add a payment card

I can't delete my old payment card yet

See -> Remove a payment method from your Apple ID

Remove a payment method from your Apple ID - Apple Support

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If you can't remove a payment method

If you have subscriptions like iCloud+, have an unpaid balance, or you use purchase sharing, you must keep at least one payment method on file.

• Do you have a subscription? You can cancel iCloud+ and cancel other subscriptions from Apple. When your subscription period is over, remove your payment method.
• If you're in a Family Sharing group and you use purchase sharing, the family organizer must have a payment method on file. If you're the family organizer, turn off purchase sharing, then remove the payment method.
• If you have an unpaid balance, you might not be able to remove a payment method. Instead, change your payment method.
• 👉 If you still can't remove a payment method, contact Apple Support.

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If you change Country or Region, you must comply with all regulations described here ->  Change your Apple ID country or region

Change your Apple ID country or region - Apple Support (KW)

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What to do before you change your country or region

• Spend any balance remaining on your Apple Account balance. If you're not sure whether you have a balance, check your Apple Account balance.
• Cancel your subscriptions and wait until the end of the subscription period. 
• Wait for any memberships, pre-orders, movie rentals, or Season Passes to complete, and wait for any pending store credit refunds to process. You can cancel a pre-order.
• Make sure that you have a payment method for your new country or region. See what payment methods you can use.
• Redownload your apps, music, movies, TV shows, and books on any devices that you might use to play or read them in the future. Some types of content might not be available in your new country or region. Learn what types of content and services are available in your new country or region.

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You need to ZERO your Apple Account balance, it is depending on the amount:

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If your balance is less than the cost of a single item, contact Apple Support directly, they will help you clear your Apple Account balance: Get Support

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See -> Check your Apple Account balance

Check your Apple ID balance – Apple Support (UK)

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If you have Apple Account balance you can't use.

You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards or unused Apple Account balance for cash, except as required by law. If your jurisdiction allows the refund of redeemed gift balances, you can contact Apple Support to request a refund. 

When you contact Apple, include the following information:

•           Your Apple ID.

•           The refund amount.

•           Your current delivery address. 

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You need to Cancel your subscriptions and wait until the end of the subscription period. 

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Make sure that you have a payment method for your new country or region. See what payment methods you can use.

You need to have a debit / credit card issued by a bank in your new country.

You need to have a billing address in your new country.

If you're having trouble changing the region in the App Store or adding a new payment card, there could be several reasons for this issue. Here are some steps you can take to troubleshoot and resolve the problem:

1. Check App Store Region Restrictions: Keep in mind that changing the region of your App Store may have certain restrictions, especially if you have an active subscription or credit on your account. You might need to cancel any active subscriptions and use any remaining credit before you can change the region.

2. Payment Card: If you're unable to add a new payment card, it might be related to the region change. Some regions have specific payment methods that are accepted. Make sure the payment card you're trying to add is valid for the region you're switching to.

3. Check Apple ID Country/Region Settings: Make sure you're signed in with the correct Apple ID that you want to update the region for. To change your country/region settings:

- Go to "Settings" on your iOS device.

- Tap on your name at the top of the screen.

- Tap on "iTunes & App Store."

- Tap on your Apple ID at the top of the screen.

- Tap on "View Apple ID."

- Tap on "Country/Region" and select the new region you want to change to.

4. Payment Information: Sometimes, an issue with your payment information can prevent you from changing the region or adding a new payment card. Ensure that the payment card details you're entering are accurate and up to date.

5. Apple ID Balance: If you have an Apple ID balance, it might need to be used up before you can change regions. You can spend your balance on apps, music, movies, or other content.

6. Contact Apple Support: If you've tried all the above steps and are still experiencing issues, it's a good idea to reach out to Apple Support for assistance. They can provide personalized guidance based on your specific situation and help you resolve the issue.

7. Consider Restrictions: If you're using Family Sharing, the family organizer can set certain restrictions for family members. Make sure that the region change isn't being restricted by these settings.

Keep in mind that some changes, such as switching between countries or regions, can have implications for your App Store purchases, subscriptions, and content availability. It's important to understand the potential impact of changing regions before proceeding.

If you're unable to delete your old payment card, it might be due to existing subscriptions, unpaid balances, or other account-related factors. If the issue persists, contacting Apple Support is the best way to get personalized assistance in resolving the problem.