Hi there elkie54,
Thanks for reaching out to Apple Support Communities. It sounds like you are the organizer for your Family Sharing group and are running into an issue with an adult member’s account, when he tries to purchase music. We’ll be happy to look into this with you.
Let’s start with the steps on this page: Family purchases and payments. Be sure to check the Apple ID he’s using and make sure it’s the right one. Make sure Purchase Sharing is enabled.
If the issue is unresolved, let us know the following details:
-Did you notice if the code was specifically about asking to buy the song or album?
-If he buys or downloads a free app, does he have to send a code to your iPhone?
Best regards.