Canvas Quizzes do not have a true extra-credit feature for automated grading. Extra credit is possible, but requires manual grade input as shown below. For large classes that need a more automated workflow, see the workaround described at Creating an Extra Credit Quiz .

To create an extra credit question within a classic quiz, set that question’s point value to zero. After the quiz is submitted, assign extra credit points manually in SpeedGrader. Any points you award will be added to the quiz total score, and the total can exceed 100%. Once entered in SpeedGrader, the updated scores transfer automatically to Grades.

How do I create a classic Quiz with an extra credit question in Canvas?

Create a new quiz with the Classic Quiz engine, adjust the settings, and save it.
Select Edit, and then open the Questions tab.
Enter each required question and its point value, selecting Update Question after each.
To add an extra credit question:
Create the question and enter the point value to zero (1).
We recommend letting students know that extra credit points will not be reflected in their score until the question is manually graded (2).
For your own reference, you can give the question a title (3). Question titles are only visible to instructors.

Extra credit quiz question setup. Question title, question points, and question prompt highlighted

How do I grade extra credit quiz questions in classic Quizzes?

After students complete the quiz, open SpeedGrader and add points as needed to each correct extra-credit question (4).
Select Update scores (5) to save these scores to Grades. The score can exceed 100%. Enter optional feedback in the Comment field and select Submit (6).