Setting up Turnitin assignments work differently than regular assignments. This type of assignment is not created using the Assignment Module.
To create a turnitin assignment, make sure you have a builders role by choosing the build tab near the top left of the screen. From the Course Tools menu on the left of the screen choose Course Content. The Turnitin assignment is built in the course content window as opposed to the assignment window.
At the top of the page you will notice a button called Add Content Link. Click on this button to access the assignment options.
From the options on this menu choose Turnitin Assignment.
Upon choosing this option you will notice a button called Create Turnitin Assignment. Choose this option and you will be prompted to title your assignment and then create it.
Upon the assignments creation you will be prompted to choose options for the assignment. Notice that at this stage the options reflect an actual interaction with turnitin.com.