Turnitin allows you to resubmit your assignment right up until the time the assignment is due. The newly submitted file will replace the old one. Once it goes past the due time (usually 11:59pm on the due date) you cannot resubmit.
However: the first time you submit, you will receive an Originality report. The second and subsequent times you will receive an Originality report with a 24 hour delay. (This is how Turnitin is set up - so that people can't keep resubmitting trying to "beat" the system by getting a report each time.)
If you find Turnitin doesn't allow you to resubmit and it's before the due date, check the due time on Moodle. The lecturer may have inadvertently set the time due incorrectly. If so, please contact your lecturer for this to be fixed.
(Edited by Gayle Kent - original submission Thursday, 19 March 2015, 4:14 PM)