If you edited the assignment settings, and set the allowed attempts to unlimited (as shown below), to find that your students can only submit once to the Turnitin assignment, please note that for external tools, you should ignore the assignment settings for submission attempts, moderated grading, anonymous grading and anonymous instructor annotations.
Follow the instructions below instead.
Access Turnitin Assignment Inbox settings
Click the cog icon on the top right to access the Turnitin Assignment Inbox settings.
If you see the collapsed menu icon (three horizontal lines) instead, click that, then Settings.
Scroll to Similarity Report header and set Similarity Report generation
There are three options for Generate Similarity Reports for student submission.
Most users will choose Generate reports immediately (students can resubmit until due date): After 3 resubmissions, reports generate after 24 hours.
If you do not want to allow resubmission, choose Generate reports immediately (students cannot resubmit).
You can also choose to Generate reports on due date (students can resubmit until due date).
Notwithstanding when the reports are generated, you can also choose whether the students can view the Similarity Report.
Check Allow students to view Similarity Reports, in order for them to do so. Uncheck to prevent them from viewing the report.
For further information on the Optional Settings: Configure the Turnitin Assignment Inbox settings.