An Invoke outbound Web service node in the flow is associated with an interface operation that does not contain an input message, or the Invoke outbound Web service node does not have an interface operation associated with it.
Open the .wsdl files in the Operations folder of your outbound Web service subproject with the operations editor until you find the operation that is missing an input message. Add an input message to that operation, save the operations file, and run the Generate Runtime Code wizard again.