Thanks for posting in the Spotify Community, and welcome!
If the code on your gift card isn’t legible, I would recommend reaching out to Customer Support; they’ll take a look and do their best to help you to redeem your gift card.
You can find the link to contact Customer Support here (where the page reads “Contact us”).
Let me know if you have any other questions, and have a good day!
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Note: I'm not a Spotify employee.