It would be difficult to provide a solution without understanding the shipping details you have set up. I would suggest you contact Shopify support via chat to get a faster response.
Thanks for your question! There are definitely a few factors we generally look to which can be the cause for a shipping error such as the one below.
To rule out some of these factors, you can:
From there, if you're still running into issues at checkout, you're also very welcome to reach out to our support team via our Help Center so that our team can take a closer look. Thanks!