Cannot Convert Adult Account to Child Account - Server Error

I am unable to convert an adult account to a child account. The adult account is part of a family sharing group. When following the entire process of changing the birthdate and Approve Request email for the family organizer, I receive the error "Cannot change date of birth. The date of birth for "Name" count not be changed because of a server error. Try Again" I have contacted Apple support twice with no resolve. Has anyone else been successful at resolving this issue?

SOLVED:

you have to tap on authorization link by your mac . You will see another error: "A VALID PAYMENT METHOD IS REQUIRED TO UPDATE XXX'S DATE OF BIRTH. a valid card or mobile phone billing method confirms you can provide parental consent".

Enter to your personal account and manage payments. Modify and insert a valid credit card . Give ok, retry the procedure and you will able to modify your son date of birth !