Family payment

He must owe a payment from his previous subscription. He may be in the family but is not utilizing the family membership if there is an outstanding payment on the account. Call xbox support or contact chat support and clear the payment and then re-add the
account to the family pack

How can i give money without that popping up

"Money can't be transferred to another Microsoft account and can't be converted from one
currency to another."

Here's info on how to add money to a child's account at this

link. I hope this helps!

It seems then you should be able to add a payment option if your not in on a family plan, Is something wrong with the region of the payment and your account?

To the above is which I'm guessing they canceled the Family Plan. Every Account now is simply a "family plan" without any further payments.