Hi Jelyvin,
Thanks for using Apple Support Communities. It sounds like you are trying to set up Apple Cash for a child in your Family Sharing group. Right?
This is expected behavior. As mentioned in Send and receive money with Apple Cash, "To send and receive money with Apple Cash, you must be at least 18 years old and a resident of the United States. If you're under 18 years old in the United States, your family organizer can set up Apple Cash for you as part of Apple Cash Family. Then, you can send and receive money with Apple Cash."
In order for minors in your Family Sharing group to use Apple Cash, make sure you take these steps from Use Apple Cash and Apple Card with Family Sharing on iPhone (U.S. only).
"Set up Apple Card Family
The family organizer can invite one eligible member of their Family Sharing group who is 18 years or older to co-own Apple Card. Members of the Family Sharing group who are 13 years or older can be added as participants.
- In Wallet
, tap Apple Card. - Tap
, tap Share My Card, then follow the onscreen instructions."
Take care.