I can't redeem my gift card on my laptop computer.

See -> How to redeem your Apple Gift Card or App Store & iTunes gift card

How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)

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Redeem a gift card on a Windows PC

1. Find the 16-digit code on the back of the card. For some gift cards, you might need to peel or gently scratch off the label to see the code.
2. Open iTunes.
3. From the menu bar at the top of the iTunes window, choose Account.
4. Choose Redeem.
5. Sign in and follow the instructions on the screen.