I have tried printing a Amazon Buy Shipping label and am seeing an error returned:

ShipmentAlreadyExists: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx


This error is being returned because the shipping label has already been created for this consignment. If it has not been generated in Linnworks, then it means that shipping label has been already generated on Amazon.

Shipping label needs to be either cancelled from Amazon for it to be printed from Linnworks or it can be printed from Seller Central directly.