Turnitin has changed the way it connects with OWL and in order to accommodate this change, the integration in the Assignments tool has been rewritten and implemented in the latest OWL update, released August 29, 2016

If you have created any OWL assignment(s) using Turnitin before August 29th 2016, it is recommended to re-create those assignment(s) to minimize potential service interruptions with Turnitin. This includes OWL assignments that were duplicated or imported from another course site or a project site. 

Any OWL assignments created, duplicated or imported on or after August 29th 2016 don't need to be re-created. Assignments that are not using the Turnitin service don't need to be re-created. 
For more information about this change or how to re-create assignments, please visit http://uwo.ca/its/elearning/news/2016/update_turnitin_integration_in_owl.html