I'm getting a 'INVALID_MSKU_REMAP' error when trying to add items to a shipment

The reason why you're getting this INVALID_MSKU_REMAP error when trying to add items to a shipment is that there's already a different item in your inventory on Amazon with an identical SKU.

Please check for duplicates on your Seller Central account by looking for the SKUs that return this error. To add these items to a shipment successfully in Seller Engine Plus all you need to do is change the SKUs then try again. You can do this manually or by re-assigning SKU values.