Similar to the blocked by Andon cord manager this error is fulfillment center performance issue.
The error is by Asin and will not allow a checkfba to be completed or the item to be added on to a shipment.
Create a test shipment for this item through seller central to get a reason why the Asin is being restricted. Recorded cases of the error were due to a shipment performance issue on the Asin. (such as unreadable barcode)
View the last shipment the sku was placed on as this is most likely where you can view the issue.