The app requires the Calculated Rate feature to be enabled on your shop in order for it to connect to your zones and show rates at checkout.
Normally, there will be a big red banner at the top of the app, like this:
But, in rare instances the banner may not show.
How to see if the Calculated Rate feature isn't enabled.
Go to Shopify Admin > Settings > Shipping >
Then scroll down to the bottom of the page to Accounts and intergrations, and click Manage Integrations
If you see the panel with the truck illustration saying Enable third-party calculated rates at checkout then it means you do not have the feature enabled.
Click the link below to learn how to enable the Calculated Rate Feature on your shop