Refer to Create Quiz as Document instructions before completing these steps.
Select Library to see your quiz listed as content.

Select (1) Assignments to assign your quiz. Then, select (2) +Add Assignment.

Select Content to choose your content to add to the assignment. You will be selecting your quiz.
Next, you will be able to customize your Assignment Name and Submission Deadline settings under Basic. Then, select Next Step.
Make sure you input a Submission deadline.