In the My Studies Course management panel  select Course Tools> Turnitin Assignments

Select the submission point and open it so you can see the assignment inbox.

Select Roster Sync

That’s it.

Who does this?

Turnitin lives outside of My Studies and every now and then the two get out of step and need a prod.
Roster Sync is a way to force Turnitin to update the list of students it holds for a module.