You'll see this error message generally for one of two reasons:
- You're trying to checkout with a shipping address that contains a country for which you've disabled shipping. To check which countries you've enabled shipping for, login to your store's control panel and click the Settings -> Shipping Settgs menu option. Make sure your country is enabled in the Available Countries list, or just choose -- All Countries -- to ship to all countries. Click Save and you should now be able to checkout.
- You've selected one/more shipping providers but haven't entered physical dimensions for the products in your store. Most shipping providers require that every product in your store has physical dimensions (width, height and depth) setup in order to provide you/your customers with real time shipping quotes. To add physical dimensions for a product, simply login to your store's control panel and double click the Products tab. Click Edit next to the products you want to add physical dimensions for, and type in the dimensions under the Shipping Details section towards the bottom of the page. Click Save and repeat for as many products as necessary. You should now be able to checkout.