Unfortunately, this error is coming directly from Amazon. There is a product in your shipment that is causing a conflict with the inbound shipping and/or the fulfillment center, but Amazon does not give us any indication as to the specific issue. 

We have contacted Amazon and they suggest each seller experiencing this issue contact Seller Support for more information on the error.  Please let them know that you are listing said item(s) via their MWS API and are receiving the following message: (insert the text of YOUR SPECIFIC ERROR MSG HERE what you see below is an example)

Invalid Items found - ShipmentId [FBA53HXYFX] MerchantCustomerId [A2WOJIKNACJWE4] InvalidItems[ (skuType=MSKU, sku=2016-11-30-B0006SGYS8-1099, reason=NOT_ELIGIBLE_FC_FOR_ITEM)]Shipment Id: FBA53HXYFXRequest Id: fb04a1f9-721f-4be3-a9c7-ba4063c13f69