This is almost always due to a typo. Remember that Enrollment Keys are case-sensitive . Ensure there are no accidental spaces before or after the numbers/letters.
If the information appears correct but you still cannot enroll, the class may have expired or been reset by the instructor. Turnitin classes have end dates. If the course end date has passed, the system blocks new enrollments. In this scenario, the student cannot resolve the issue alone and must contact the instructor to either extend the class end date or provide a new Class ID for a new section. class id and enrollment key for turnitin