Only course masters have the ability to view and edit an instructor's access levels. For more information on course masters, please read the help article Are there different types of instructor enrollments?.
You will need to enroll the instructor before you can edit their access levels. For more help with this, please read How can I enroll a user in the course as an instructor or TA?.
Hover over the instructor enrollment of interest and click "See access levels"
If you do not see this link, then your current enrollment does not have sufficient privileges to view/edit instructors' access levels.