Why can’t I change my country

i got a us gift card but I’m not from the us

See 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

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Code must be redeemed in a different country or region

If you see a message that says the code must be redeemed in another storefront, the card was probably purchased in another country or region. You can't redeem Apple Gift Cards or App Store & iTunes Gift Cards outside of the country or region of purchase. For example, if you bought an App Store & iTunes Gift Card in France, you can't redeem it in the App Store in the United States.

If you need more help, contact Apple Support.

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I change my country to us o can’t buy anything because it needs USA credit card

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. 

2.

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

3.

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.