Cause
Your Cengage course already has students enrolled.
Students must be enrolled through the LMS in order for their grades to be synced. You cannot link a Cengage course that already has students enrolled.
Resolution
Either drop all students from your course, or create a new Cengage course by copying the existing course.
Copying your course allows you to create a course with no enrollments without losing any activities or gradebook settings. Any scores from the original course will be lost and must be manually entered in the new course.
Instruct your students to enroll in the course through the LMS.