‘Rate ToState and Destination Address State field must match’

This error message usually comes up in one of the two situations described below:

a. when creating a shipping USPS label,

b. when creating a return label

This means that one of the addresses involved doesn’t have a correctly formatted State field. Please note that USPS requires a two-letter state code, and both letters must be capitalized. 

Once you’ve changed this and saved your changes, you should be able to create your labels with no issue.