Seems to be OK, the only thing that needs to be completed is the REMOVE function, which also needs to delete the corresponding exercise of a given lesson:

"Each time you Swap or Remove a lesson, you should do the same with the Exercises, if there are any, which follow the lessons."

Remove:

                else if (firstCommand == "Remove")
                {
                    lessons.Remove(lessonTitle);
                    lessons.Remove(lessonTitle + "-Exercise");
                }

Maybe check also your SWAP method with other solutions from the forum, just in case that some other judge-errors come up.

Swap-Demo:

static List<string> SwapOperator(string title1, string title2, List<string> courseSchedule)
        {
            bool course1Exist = courseSchedule.Exists(course => course == title1);
            bool course2Exist = courseSchedule.Exists(course => course == title2);
            bool exercise1Exist = courseSchedule.Exists(course => course == title1 + "-Exercise");
            bool exercise2Exist = courseSchedule.Exists(course => course == title2 + "-Exercise");
            int course1Index = courseSchedule.FindIndex(course => course == title1);
            int course2Index = courseSchedule.FindIndex(course => course == title2);
            int exercise1Index = courseSchedule.FindIndex(course => course == title1 + "-Exercise");
            int exercise2Index = courseSchedule.FindIndex(course => course == title2 + "-Exercise");


            if (course1Exist && course2Exist)
            {
                string oldElement = courseSchedule[course1Index];
                courseSchedule[course1Index] = courseSchedule[course2Index];
                courseSchedule[course2Index] = oldElement;
                course1Index = courseSchedule.FindIndex(course => course == title1);
                course2Index = courseSchedule.FindIndex(course => course == title2);

                if (exercise1Exist && exercise2Exist)
                {
                    string oldElement2 = courseSchedule[exercise1Index];
                    courseSchedule[exercise1Index] = courseSchedule[exercise2Index];
                    courseSchedule[exercise2Index] = oldElement2;
                }
                else if (exercise1Exist)
                {
                    string oldElement4 = courseSchedule[exercise1Index];
                    courseSchedule.Remove(oldElement4);
                    courseSchedule.Insert(course1Index + 1, oldElement4);
                }
                else if (exercise2Exist)
                {
                    string oldElement5 = courseSchedule[exercise2Index];
                    courseSchedule.Remove(oldElement5);
                    courseSchedule.Insert(course2Index + 1, oldElement5);
                }

            }
            return courseSchedule;
        }

Best,