In https://help.turnitin.com/feedback-studio/blackboard/lti/instructor/assignment-inbox/submitting-on-behalf-of-a-student.htm - it notes that you can't submit on behalf of a student, if it's set to anonymous. I agree that you can't if you use those 3 dots at the right, however, I advise all staff to use the drop down at the top - as that works for both anonymous & named students.