Had the same problem with my solution, Test #8 error is triggered by the 'Exercise' operation which requires to check whether a given course AND it's exercise exist or not or just one of the two.

Exercise-operation (88%):

        static List<string> ExerciseOperator(string title, List<string> courseSchedule)
        {
            bool courseExist = courseSchedule.Exists(course => course == title);
            int courseIndex = courseSchedule.FindIndex(course => course == title);
            int exerciseIndex = courseIndex + 1;
            if (courseExist)
            {
                courseSchedule.Insert(exerciseIndex, title + "-Exercise");
            }
            else if (courseExist == false)
            {
                courseSchedule.Add(title);
                courseSchedule.Add(title + "-Exercise");
            }
            return courseSchedule;
        }

Exercise-operation (100%):

        static List<string> ExerciseOperator(string title, List<string> courseSchedule)
        {
            bool courseExist = courseSchedule.Exists(course => course == title);
            bool exerciseExist = courseSchedule.Exists(course => course == title + "-Exercise");
            int courseIndex = courseSchedule.FindIndex(course => course == title);
            int exerciseIndex = courseIndex + 1;
            if (courseExist && exerciseExist == false)
            {
                courseSchedule.Insert(exerciseIndex, title + "-Exercise");
            }
            else if (courseExist == false)
            {
                courseSchedule.Add(title);
                courseSchedule.Add(title + "-Exercise");
            }
            return courseSchedule;
        }

Best,