Can you check you have asked Shopify Support to add the 'Carrier Services API' to your Shopify plan. You can email Shopify at email@example.com. This is a Shopify requirement for merchants wanting to display real time shipping quotes at checkout.
If you do have Carrier Service API on your Shopify plan but you had our app installed beforehand, you will need to uninstall our app and then reinstall it again to have it work correctly (you won't lose any data or information you have already added to the app if you do this).
You can find great tutorial videos on how to setup our Shipping Unlimited app correctly in your Shopify store at ShippingUnlimited.com.au.