Q: family sharing payment method error

is that content bought before u arrange family share or after. You can check which contents u can share from here Which purchased content can I share using Family Sharing - Apple Support Also if issue persists u may contact with AppleCare iTunes store department to see if there is any issue with account. But mentioned that main account is you wife's so she may need to contact with them

Well, you must be sure that "you try make purchase." using credit card, not debit card.

What I can tell you to try is that "you should check the account details." typed on Apple ID. Go to settings > iTunes&App Store and tap on Apple ID. View the account. Probably Apple ID will ask you to change some of info, most probably password.!

Besides, you can do the same steps using iTunes on PC. If you sure that credit card works fine, except family sharing, as I said you must see the account details and change info there. Let's start trying the steps by changing your password.