Here what you should know about Activity Completion:
Activity Completion reports are related to the activities that are inside courses in your LMS. Activity completion allows the teacher to set completion criteria in a specific activity's settings. A check (tick) appears against the activity when the student meets this criterion. The criterion might be viewing, receiving a certain score, or a student marking it as complete.
First, you will have to make sure that Completion Tracking is enabled site wide. Please find more information here.
Second, make sure to enable Completion Tracking for courses. From Instructor Account go to Your Courses > Edit Settings > Enable Completion Tracking:
Third, after Course Completion tracking is enabled you can go to your Activities to set up the criteria for Activity Completion. Go to your Courses > Activity > Edit > Activity Completion setting:
Here you can find detailed information about Activity Completion, its settings and usage.
Activity Completion Reports: