I just tried and it should work.
- I have a user with the username "m1".
- I went to the Site administration > Users > Permissions > Assign system roles and I assigned the role "Manager" to this user.
- I logged in as the user m1
- I went to a course
- I went to the calendar
- Pressed "New event" button
- The field "Type of event" is pre-selected with the "Course" and I can create course-level events.
Are you sure your manager user is inside a course when created the event? What Moodle version are you using?