If you click on the turnitin assignment then click the button in the top right that says "Update this Turnitin Assignment" you will be taken to the main settings for the assignment.
Scroll to the "Turnitin Advanced Options" section and you can choose to allow them to submit their papers repeatedly until the due date. This enables them to use the spelling check as well as originality report generated then re-submit to overwrite the previous report up until the due date so they can improve their papers on their own.
