Full Error Message:
Proposed Shipment Name: Double sausage sampler 2018-12-04 10:19, Prep Type: SELLER_LABEL, Destination: MEM1
Error: You must include a valid ShipmentId with a call to the CreateInboundShipment operation. Get ShipmentId values by calling the CreateInboundShipmentPlan operation. The request to CreateInboundShipment must include only items and quantities that have been previously planned through CreateInboundShipmentPlan. If a ShipmentId is not used to create a shipment within 48 hours it will expire.
Solution:
The error is basically saying that shipping plan was already used to create a shipment.
Have the user check to see if the shipment is showing in Seller Central. If so, click on Sync Shipments to see if the shipment then displays in IL. It hasn't worked for me when I've tried it so I don't hold out much hope for this to work.
If they have Box Content ON:
The shipment in SC won't allow them to add BC there so they will have to delete the shipment in SC, then come back to IL and close the batch, restore the batch, then process again to get to the Proposed FBA Shipments page. This works well if that was the only shipment from the batch.
If it was one of several shipments, and the other shipments are created, then the user should delete all shipments from the batch in SC, click on Sync Shipments to remove them from the Working Shipments window, close the batch, restore the batch and go through the process again.
If they have Box Contents OFF:
They can click Decline for that shipment with the issue and finish out any other shipments needing creation in IL and close their batch as usual to finish their shipment in seller central, as usual