One of the things you can check that is a common cause for this error is your shipping locations. You will want to ensure that these are activated. This setting is found at the bottom of the shipping rates page. Otherwise, as mentioned by @Lucid_Polygonthis post is had a comprehensive troubleshooting list of possible causes.
If you are still having issues after going through those possibilities, Please visit our help center to create a support request.
If there is anything else I can help you with, please let me know.