We don't know why, but there seems to be TWO locations to add in a billing method.
First, go to business.facebook.com and login.
Then There Are Two Locations to Add in Your Billing Information:
1 - Clicking Billing & Payments on the Left.
2 - Go to the Ad Manager, and then add payment method.
You will have to look around, there are a couple places you might be prompted to add in your billing information.
But the other place to add your billing information is the Ads Manager page.