This issue can occur when ShipStation throws a 404 error while creating/updating an order (SSOrderAPI.java:619)
ShipStation throws this error in 2 scenarios
If multiple orders are mapped to the same order in Shipstation i.e. all those orders have been merged in ShipStation. So when Chargebee tries to find the record during sync it throws a 404 error.
If shipped, then you can click that Ignore button on the ShipStation Sync window to ignore these records.