This is a generic error message that displays at checkout when a shipping method is not available for the order -- the reference to 'United States' just means that's the country from the address you're using for your testing. The error could be caused by one of a few different reasons. I would advise reaching out to Shopify support directly to see if they can help you identify why a shipping method isn't being triggered at checkout. Additionally, you should try testing checkouts from addresses in Canada and Mexico (countries in the same Zone as the US), as well as countries in your Rest of the World zone to see if you get the same error. If you get it in Domestic but not ROW, the issue is likely related to the Zone itself.