How to add an extra credit assignment?
No one can give a simple answer to this question without additional information. It depends on how your course is set up.
For example, if you're using weighted assignment groups, then extra credit isn't extra credit until students have grades in assignment groups where the weights add up to be more than 100%. This is where waiting until the end of the term comes in. Another option is to make an assignment group to hold the extra credit, but make it worth 0% of the grade until students have grades in all assignment groups.
If, on the other hand, you're not weighting assignment groups, then you can just make an assignment worth 0 points and it will count as extra credit without needing to wait until the end of the semester.
Making quizzes extra credit may involve going through to each quiz and adding fudge points as there is no way to mark a question as extra credit in legacy quizzes.
Here is Canvas' lesson from the Canvas Instructor Guide: How do I give extra credit in a course?
Searching the community (use the search tool at the top of the page) for extra credit related to the type of extra credit you need can provide lots of additional information and suggestions.
Create an assignment with 0.1 point value and a due date for the end of the term. Save and publish. Add whatever value you want in gradebook. For example, I give 5 points for attendance at weekly office hours. After each session, I increment each attendees extra credit total by five points.
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