Select a Turnitin assignment you wish to enable resubmission in.

Select "Settings" in the Turnitin Assignment header options.

Expand the "Similarity Report Options" section.

In the "Report Generation Speed" setting, decide whether you want to allow maximum 3 submissions or unlimited submissions.

Generate reports immediately (students cannot resubmit)
Generate reports immediately (students can resubmit until due date): After 3 resubmissions, reports generate after 24 hours
Generate reports on due date (students can resubmit until due date)

Turnitin Assignment Resubmission