Some folks have noticed that their high internet Firewall settings are blocking some actions.
Credit card purchases maight be included in that limitation.
or
Call Customer Support and see if they can find what the problem is
(You are required to have a US credit card with a US Billing address...if that's the issue)
Had the same issue. Called support - no clue. Researched it on Reddit and one of the suggestions was to use "123 Main St" as the street address as part of your billing info when purchasing. It worked! Unbelievable. Mentioned to the customer service that I used a fictitious address to address this and she was unfazed. Doesn't give you confidence in the tax calc abilities...