I have tried printing a (New) Amazon Shipping label for an OFF-Amazon order and am seeing an error returned:
InternalFailure: We encountered an internal error. Please try again
This error is usually returned for OFF-Amazon orders when the Shipper has either not been whitelisted to ship non-Amazon orders, or if the Return Address has not been filled in on Amazon end in Seller Central. Please confirm that the whitelisting has been applied for your account for OFF-Amazon Shipping and that the Return address is specified in Seller Central for Amazon Shipping.
If both options have been confirmed, but the error persists, please contact Linnworks Technical Support.
Above error may also be returned for ON-Amazon shipments, in which case you would also need to contact Linnworks Technical Support for a further investigation.