When there are no rates at checkout, Shopify will show a piece of static text that can be edited.
It looks something like this, by default:
The text normally says 'There are no shipping methods available for your cart or address.
This is a single piece of static text that will appear under any circumstance when there are no rates available.
Here is how you can edit this text:
Go to Shopify Admin > Settings > Checkout > Manage Checkout Language
Scroll to the bottom of the Checkout settings
Click Command/Cntl + F in your browser to bring up the mini page search window, and start typing 'No rates for cart or destination notice' to find the input field.
Enter in the text you want to show instead:
Then save your changes.
It will look like this at checkout when there are no rates:
Remember, you can only go as specific as you have cases where you have no rates showing.
If there are multiple reasons why no rates show, then you should consider a more generic message like:
"There are some products in your order that cannot be shipped to your location."