i can't change my gamer tag

You will need to check your region settings on your PC

do you can buy xbox live credit by an retailer, then you don't need an credit card.

is your credit card locked to a region? your billing address must match of your credit card

Through the Xbox.com website, scroll to the bottom of the page and click on the tiny globe icon and select your region.

First please change your Xbox and Microsoft account to your country (South Korea I believe): This is needed to use a credit card from your region.

Xbox one:

Press the Xbox button to open the guide. Select Settings. Select System. Select Language & location. Select your new location from the list, and then select Restart now.

Microsoft account (for billing):

https://live.xbox.com/accountmigration Please read all the warnings on this page before you migrate to your country that you're in.

You will need a supported payment method to pay for a gamertag change (credit/debit/paypal).

Once you have migrated your Microsoft Account you can add your credit card to your profile here:

https://account.microsoft.com/billing/payments

Also, make sure your address is correct on this page as well:
https://commerce.microsoft.com/PaymentHub/Profile

If you can't use one of those payment methods, I would suggest a prepaid currency card you can buy at a store in the gift card section and then redeem that on your console on the store or at
https://redeem.microsoft.com/enter
And then you can change your gamertag with that money in your account. Make sure you only use one that was made for your region or else it won't work.

i tried gift card section KR so i can't used that

First please change your Xbox and Microsoft account to your country that you are in: This is needed to use a credit card from your region.

Xbox one:

Press the Xbox button to open the guide. Select Settings. Select System. Select Language & location. Select your new location from the list, and then select Restart now.

Microsoft account (for billing):

https://live.xbox.com/accountmigration Please read all the warnings on this page before you migrate to your country that you're in.

You will need a supported payment method to pay for a gamertag change (credit/debit/paypal).

Once you have migrated your Microsoft Account you can add your credit card to your profile here:

https://account.microsoft.com/billing/payments

Also, make sure your address is correct on this page as well:
https://commerce.microsoft.com/PaymentHub/Profile

If you can't use one of those payment methods, I would suggest a prepaid currency card you can buy at a store in the gift card section and then redeem that on your console on the store or at
https://redeem.microsoft.com/enter
And then you can change your gamertag with that money in your account. Make sure you only use one that was made for your region or else it won't work.