Apple does not permit 12 years olds to add payment methods. Many states have banking laws that restrict the legality of minors having access to debit and/or credit cards.
>>How children and teens can use Apple Cash
If your family organizer set up Apple Cash for you, you can use it to make purchases in participating stores, apps, and websites.
You can also use Apple Cash to send and receive money with people you know. If you're under 13 years old, you can't add a debit card to Wallet to use with Apple Cash.
Apple Cash services are provided by Green Dot Bank, Member FDIC. Learn more about the Terms and Conditions. Only available in the U.S. on eligible devices. To send and receive money with an Apple Cash account, you must be 18 and a U.S. resident. If you’re under 18, your family organizer can set up Apple Cash for you as part of their Apple Cash Family account. Security checks may require more time to make funds available. Apple Cash Family accounts can send or receive up to $2000 per transaction or within a seven-day period. Sending money from Wallet requires iOS 15.5 or later.<<
Set up and use Apple Cash Family - Apple Support