I have tried printing a FedEx shipping label and am seeing an error returned:
3050: Invalid Recipient Postal Code Format
If this error is returned, you need to verify if the postal code is in the correct format as expected for the destination country where order is being shipped. In some cases the postal code may include extra spaces or characters which are not expected by courier and thus result in the above error.
Recipient postal code can be edited in Linnworks.net > Orders > Open Orders, when you Edit the order and go to Shipping Address section.