In order to use this app you will need Carrier Calculated Shipping API access enabled on your Shopify account. This is included with both the Shopify Advanced and Shopify Plus plans.
If you are not on either of these plans, don't panic!
You can easily add Carrier Calculated Shipping API access to your existing plan for an additional $20 USD per month. Or if you switch to paying Shopify annually they will provide it free of charge.
Just contact Shopify Support and request they add it to your plan.