Not being able to use own credit card in family sharing

1. When in Family Sharing, and with Purchase Sharing turned on, only the Organizer's payment method is charged. This is specifically outlined in the Family sharing setup process as it's designed for under age children first and foremost that have no payment method. Adult usage is secondary.
   1. click here ➜ How to share apps and purchases with your family - Apple Support
2. If a family member has gift card credit balance on their Apple Account, that will be used for purchases automatically before charging the organizer's payment method, but it may still require verification of the Organizer's payment method first if there is some issue with it, before using the balance in the family member's account. Once it's verified, the account balance should always be used before charging the Family Organizer's payment method.
3. Purchase Sharing can be turned off as shown in the link above so each member can use their own payment methods, but it is turned off globally for all members of the family sharing group.
4. If you don't like the way Apple has this set up, you can offer Apple Feedback at the following website. If enough people do, they may consider changing it.
   1. click here ➜ Product Feedback - Apple

You are free to have the family organizer turn it off, for the specific purchase, and then turn it on again if you want.

Outside of that, as said, it will use your account balance first always, regardless of of wether its turned on or off, so should not be an issue if you have enough Apple account balance to cover the purchase.

The only time the organizer's payment method would need to be verified is if there is a specific issue with it.

It should nor require verification or a code of any kind as long as its active and verified.