This is the updated Version

[makikovaughan]

What I have modified:
In Student.java
Initially I thought I had to use approvedCourses to add the course information.
So, I added courses.add(course);
Also, I added if statement.
public void enrollToCourse(Course course )
{
    //TODO implement this method
    if(!(isAttendingCourse(course.getCode()))) {
        //Initially I did correctly. I overthought add all registered courses to approvedCourses.
        //Modified back again
        courses.add(course);

        //Used for isAttendingCourse
        registerApprovedCourse(course);
    }
    else {
        System.out.println("The student has been enrolled");
    }
}
 
 
In StudentServices.java
I created a method to be able to print the courses, but reflected the duplicate courses.
 
public void addEnrollStudents(String studentId, Course course) {

    if ( (enrolledStudents.containsKey( studentId ) ))
    {
        ArrayList courseList =  enrolledStudents.get(studentId);
        System.out.println("CourseList" + courseList);

        //For avoiding ConcurrentModificationException,used for loop
        boolean flag = false;
       for(int i = 0; i < courseList.size();  i++) {
            if (courseList.get(i).getCode().equals(course.getCode())) {
                flag = true;
            }
        }
       if(!(flag)) {
           enrolledStudents.get(studentId).add( course );
       }
    }
    else {
        enrolledStudents.put( studentId, new ArrayList<>() );
        enrolledStudents.get(studentId).add( course );
    }

}