Extra credit is currently not a default option in Canvas. However, you can give students extra credit using a variety of options. One of the most common ways to give extra credit is to Create a New Assignment with Zero Point Value.
Create a New Assignment with Zero Point Value.
Create a new assignment with zero points possible [1] and select any submission type [2]. Assignments set to the No Submission type work well for classroom activities, such as in-class discussions or presentations.
After students complete an online submission or complete and submit work offline, you can add and adjust points in the Gradebook.
Note: For a zero-point assignment to factor into a student's grade, you must add a positive point value to at least one assignment in any assignment group.
Detailed instructions on giving extra credit by creating a new assignment with zero point value or by using other techniques can be found in the following Canvas guide, How do I give extra credit in a course?