Can you redeem the card after switching off your VPN?
If still no luck, follow the troubleshooting mentioned in this article, to contact Apple Support and provide them with the details:
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You might need to provide:
- An image of the full front and back of your card.
- A sales receipt showing when your card was purchased.
- The serial number from your card.
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copied from: If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support