Turnitin does not have a 'no grade' option when creating an assignment. Turnitin grades are always numerical. However, it is possible to simply not enter grades and to use Turnitin's feedback features to provide detailed feedback on student submissions only.
Why Use Turnitin for Ungraded Assignments?
If you need a similarity score (plagiarism checker) for your ungraded, formative assignment, you will need to use Turnitin.
What Happens Once 'Grades Are Released'?
At the point that grades are released to students (this is a pre-determined date defined when creating the assignment), if there is feedback but no grade, this is exactly what will be released to the student. The grade field will simply be blank. Here's how you allocate the grade and feedback.
When using Turnitin for formative assignments, it can be a good idea to activate the 'Do not store the submitted papers' setting. You can find this in 'Optional settings' in the assignment in Turnitin.