Shopify requires that all stores wanting to use third party shipping apps have the real time carrier shipping feature installed on your store. This feature is included if you are on the Advanced Shopify plan. However, if you are not on this plan, you contact Shopify directly from their help page and you can ask them to add the real-time carrier shipping feature to your current plan. They will do this for a small fee, or if you switch to the Shopify annual billing cycle. You can find out more information about this here.