I have a gift card which I would like to use to purchase a game from the App Store. The gift card has been redeemed and appears to be an option when I go to purchase. However, when I click to validate the gift card as the payment method, nothing happens. Could this be because my Mum has Family Sharing turned On, even though Ask To Buy is Off?
iPad, iOS 13
Then you need to redeem it as explained in the link above. If you already did this, then you are done. Not sure what you mean by "validate as a payment method", as once redeemed the amount is added to your account and can be used directly.
You can follow the steps here to check your new balance: View your Apple ID balance - Apple Support
Once its value is added to your account, it will be used for any purchases you make before charging your mother's payment method. You don't need to, and can't actually set it as a payment method, it will be automatically used.
Your mother's payment method may still need to be verified with its CVV code, but should not be charged.
Then you need to redeem it as explained in the link above. If you already did this, then you are done. Not sure what you mean by "validate as a payment method", as once redeemed the amount is added to your account and can be used directly.
You can follow the steps here to check your new balance: View your Apple ID balance - Apple Support
Once its value is added to your account, it will be used for any purchases you make before charging your mother's payment method. You don't need to, and can't actually set it as a payment method, it will be automatically used.
Your mother's payment method may still need to be verified with its CVV code, but should not be charged.
What kind of gift card is it exactly? : What type of gift card do I have? - Apple Support
An Apple Store gift card cannot be used for purchases in iTunes or the App store.
An iTunes, App Store, Books or Music Gift Card would need to be redeemed to your account directly to be used: Redeem App Store & iTunes Gift Cards and content codes - Apple Support
The card is an App Store & iTunes gift card.
Using Apple Gift Card with Family Sharing On
