Payment method is not accepting an American Express gift card
Payment method for I Pad is not accepting an American Express Gift Card
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You may find these support pages to be helpful:
If you need to change or update your Apple ID payment method - Apple Support
Add a payment method to your Apple ID - Apple Support
Remove a payment method from your Apple ID - Apple Support
Payment methods that you can use with your Apple ID - Apple Support
Assuming that your payment card is supported, if you cannot resolve the problem yourself, you would be best advised to seek assistance directly from Apple Support:
You can also initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store: