Very infrequently, a student gets the message "You cannot enrol yourself in this course" when trying to enroll in a course.  This appears to occur when the student account is marked incorrectly in the database.  

To fix this issue, go to the teacher tools menu and click on the Gear icon  

Go to Class Management / Class Enrollment and remove the student from the course, and then add them back into the course, making sure that they are being added as a student.

Click Back to the Dashboard

Click the Gear Icon in the teacher tools menu.

and select Run IT from the menu

Select the student's name and click the RUN button.