This usually happens either because the shipping rates currently available do not include the country/state/province that the customer is attempting to shop from, there is an error in the setup for your shipping creating a gap in coverage or there is an issue with the product weight making it ineligible for shipping based on current settings.
Our support in the community forums does not have access to your merchant admin so we won't be able to review this with you directly. I do recommend double checking all your shipping settings, product weights and default package options to see if you can find the culprit.
If you aren't able to resolve it yourself please reach out to our live support channels for further help. To access our live support, please follow this link:Shopify Help Center. Next, you will want to follow the on screen prompts to sign in, choose your topic and issue and at the bottom of the help articles provided will be all available live support options.
Shay | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog