Why can't I change the Category or Assessment Weight preferences for my gradebook?
For teachers, the grade book preferences cannot be changed if there is a Report Card mark or Progress Report mark already recorded for any student in the course section.

In the specific case where a teacher did not specify any preferences, the preferences can be added by navigating to Admin > Courses or Scheduling > Courses and using the “Gradebook Preferences” multi-action. You must have both Registrar Full and Security Full roles to access this multi-action.  Regardless which setting you choose for Update Sections Having, if no preference has not been specified for a course section, it will be added.  Note: All sections of the selected courses will be updated.
 

See also
Gradebook    Action    DAT   
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