Unable to add credit card as payment method

See -> Add a payment method to your Apple ID -> Add a payment method to your Apple ID – Apple Support (UK)

• If you want to change your card, you need to add first the new card, then remove the old card.

If you cannot add, see -> Remove a payment method from your Apple ID

Remove a payment method from your Apple ID - Apple Support

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If you can't remove a payment method

If you have subscriptions like iCloud+, have an unpaid balance, or you use purchase sharing, you must keep at least one payment method on file.

• Do you have a subscription? You can cancel iCloud+ and cancel other subscriptions from Apple. When your subscription period is over, remove your payment method.
• If you're in a Family Sharing group and you use purchase sharing, the family organizer must have a payment method on file. If you're the family organizer, turn off purchase sharing, then remove the payment method.
• If you have an unpaid balance, you might not be able to remove a payment method. Instead, change your payment method.
• If you still can't remove a payment method, contact Apple Support.

Get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

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iPhone 6, iOS 10

For sure check your iPhone model & iOS version -> Go to: Settings -> General -> About.

If your iPhone is an iPhone 6 update your iPhone to iOS 12.5.7, that is the latest iOS security update for iPhone 6.

Go to Settings -> General -> Software Update -> Download and Install (if you haven't already done that).

See -> iPhone models compatible with iOS 12.5.7 -> Supported iPhone models – Apple Support (UK)