USPS changed their “First-Class Mail Parcel” Service to “First-Class Package Service – Retail.” Merchants offering this service must change the name, as “First-Class Mail Parcel” will no longer work. Magento is working on a code update to support this change. In the meantime, information about a workaround is available at http://devdocs.magento.com/guides/v2.1/release-notes/tech_bull_USPS-patch-Sept2017.html