Im getting the same error when try to upgrade to BUSINESS plan.
There was a different billing earlier, but maybe they’re related.
Is this still happening? If so, then open a ticket for Billing:
To contact Cloudflare Customer Support, login & go to https://dash.cloudflare.com/?account=support and select get more help. If you receive an automatic response that does not help you, please reply and indicate you need more help.