If you are unable to set or change the due date of a particular activity in a course, it is likely due to the course enrollment type:
There are two enrollment types that may be defined for a course: Continuous or Range.
In a Range course, the teacher may set due dates for each activity in the course. Due dates are set via the Course Editor and are defined at an individual item level in the item Activity Settings:
In a Continuous enrollment course, due dates are calculated for each individual student based on a number of factors, most notably, the student’s start and end date in the course. In a Continuous enrollment course, teachers will not have the option to set or modify item due dates so this setting will not appear in the item Activity settings.