The cart would show an -error- for shipping if the buyer's order doesn't qualify for any of the Shipping Rules you have set up. Your current settings appear to have this sorted out already by now, so the following is primarily for others with a similar issue who may encounter this thread:
If you don't wish to ship outside the US, go to Seller Admin > Cart Shipping Settings, then in Shipping Destinations select only United States, then click Next, then click Back to Admin.
If you do wish to ship outside the US, go to Seller Admin > Cart Shipping Settings, select the Shipping Destinations you wish to ship to (or leave nothing selected to allow worldwide shipping), then click Next to proceed to the Shipping Rules screen; there, you would add at least one rule for Country:All Countries (this covers all of your chosen Shipping Destinations which don't have a rule of their own), then click Back to Admin once you're done.
This help page explains our shipping settings in detail: