I bought a gift card at the Apple store for $15
When I want to redeem it says enter valid code I already turned off the phone.
Click and check: What type of gift card do I have? - Apple Support
If you have
Apple Store Gift Cards
Apple Store Gift Cards can be used in the Apple Online Store1 or Apple Retail store. On the back of these cards, you'll find:
Apple Store Gift Cards are a solid color (gray, white, silver, or gold) on the front.
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I bought a gift card at the Apple store for $15