If a course only has one assessor, they will be responsible for all students enrolled and you do not need to assign them to the students.
However, if a course has more than one assessor, students enrolled into the course will need to be assigned one of the assessors each in order for reporting to be accessible by the assessor.
To assign an assessor to multiple students in a course:
Go to the course page
Then click on 'Reports'.
In the list of students, click the checkbox to the left of the students you would like to assign a specific assessor.
Above the list, click on the '...' button
Select '+Assign assessor.'