Adult account unable to access on Family Sharing

I can't access the family sharing page because it says "Only an adult with a valid iTunes & Apple Store payment can set up Family Sharing. Ask a parent to set up Family Sharing from iCloud Settings."

I am an adult and my birthdate is set correctly on my account.

I also have a payment method. What can I do?

[Re-Titled by Moderator]

Original Title: Family sharing not working