I can't add my Apple ID balance to the Wallet app.

Re: I can't add my Apple ID balance to the Wallet app.

Apple is now calling an Apple ID Account just an Apple Account.

If you have no funds in your Apple Account your card cannot be added to Wallet.

Check if you have the security of latest iOS version by going to Settings > General > Software updates

(Please remember to back up iPhone first if you choose to update)

Check if you are signed in to your iPhone with your correct Apple Account info:

Sign in to your Apple Account on the web or in your device settings - Apple Support

For help with adding a card to Wallet you can:

See what to do: If you can't add a card to Apple Wallet to use with Apple Pay - Apple Support

If you don't already have Apple Pay set-up, or want to add a card: See set up Apple Pay.for the details

Eg: says:

"To use Apple Pay, you need:

• A compatible device1 with the latest version of iOS or iPadOS, watchOS, macOS, or visionOS.
• A supported card from a participating card issuer.
• An Apple Account signed in to your Apple device.
• Face ID, Touch ID, Optic ID, or a passcode set on your device."

Learn if your country/region supports Apple Pay.

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More trouble-shooting, if needed:

You can check whether "Apple Account Balance" is a payment method Apple can accept in your current country or region: Payment methods that you can use with your Apple Account - Apple Support

Also: See: What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

and

How apps, content, and subscriptions from Apple are billed - Apple Support

You could try restarting your iPhone, (deletes temporary files, re-establishes connections, may update the recent setting changes, and often restores order if something is not working as expected). Then try again to add your card.

ie:

Restart your iPhone - Apple Support

Eg says: (for iPhone X, 11, 12, 13, or later)

1. "Press and hold either volume button and the side button until the power-off slider appears.
2. 
3. Drag the slider, then wait 30 seconds for your device to turn off.
4. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo."