I use apple gift card to purchase but when I use it, it always show that purchase not complete, please help

You may see various forms of a "Your purchase could not be completed" message. Some may show old URLs which will redirect to the current one which leads you to a page to select your country. Since it says to contact iTunes Support, do this:

1. Click on this link --> https://support.apple.com/choose-country-region/media-services

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.