1.
"The iTunes store is unable to process Gift Certificates at this time."
Please check the network connection:
You need to have a good Service and/or Wi-Fi.
If they are On, turn Off Hotspot & VPN.
Charge the battery > 70%.
If you can't connect to the App Store, iTunes Store, or other Apple services
If you can't connect to the App Store, iTunes Store, or other Apple services - Apple Support
-> If you tried all of the above steps and you still can’t connect, contact Apple Support.
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2.
Then try again.
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)
When redeeming a gift card, the amount will be credited to your (1) Apple ID balance and (2) you need to set your Apple ID balance as your default payment.
1.
See -> Check your Apple ID balance
Check your Apple ID balance – Apple Support (UK)
(2)
Change, add or remove Apple ID payment methods
Change, add or remove Apple ID payment methods – Apple Support (UK)
If you update your payment method or add funds to your Apple ID balance, It would take about 1 to 3 business days in order to process.
If you're still unable to update your payment method, Contact Apple Support.
