Amazon appears to be struggling mightily w/ a variety of reports over the last few weeks.
This banner has now been removed from the Ads Billing Dashboard (link):
but all functionalities remain inoperative for both of our accounts - even the ‘Learn more’ link didn’t work when the warning was still present..
My best guess is that the launch of the New Seller Central Experience/Seller Experience Lab Dashboard is throwing monkey wrenches into the works - just as occurred w/ every major Seller Central revamp since 2017.