If a student has completed all of their available submissions to an assessment, you can allow them to retake it.
Select the Tests & Quizzes tool from the Tool Menu of your site.
Select the Published-All option in the View drop-down to view the assessments that have been released to students in your site.
Select the Scores option from the drop-down menu for the assessment you would like to grade.
Alternatively, you may click on the number of student submissions in the Submitted column to view the submissions.
You'll be presented with the list of students. Underneath the name of the student who needs to retake the test or quiz, click Allow retake?
You'll be asked if you're sure you want to allow the selected student to retake the quiz. Click Retake to confirm.