If your store is not live yet and you are running a few test orders, perhaps you see the following error when you try to go into the checkout page:
This error message only appears if you have a Storefront Password in your Shopify store. This error message only appears if you have that password. So if you are not ready to go live yet, we suggest you disable the password just for a few minutes, run the test order and enable the password again. This is a Shopify limitation as they can't send information such as taxes, shipping, etc if you have a storefront password in place.