We are attempting to get unique thermal labels, which has been working since day 1 of the API, is now no longer working - odd that the error message is about carrier since this is non-partnered LTL. We've validated the package ids are what come back from get shipment packages.

GET https://sellingpartnerapi-na.amazon.com/fba/inbound/v0/shipments/xxxxx/labels?PageType=PackageLabel_Thermal&LabelType=UNIQUE&PackageLabelsToPrint=xxxxx001

Result:

400 - We are not able to fetch carrier labels while it is required. Please check the parameter request. It is possible you use incorrect number of total packages and page index.