Quick answer
Here are a few things to check…

Full answer 

1. Is the “Due Date” (both date and time) in the future? If it has already passed, the student may not be able to submit (unless your Turnitin dropbox is set to accept late submissions). If the deadline is incorrect, click the pencil next to the Due Date and modify as required.
2. Less likely, but if a student has submitted a draft and cannot overwrite it, check that you have allowed resubmissions. Make sure that the “Report Generation Speed” has been set to “Generate reports immediately, reports can be overwritten until due date” in the assignment settings.