The course has no Enroll button next to it.

If you find the course on the list, but it has no Enroll button way out on the right hand side it means the Instructor has not turned on Self Enrollment for students, but instead has Instructor enrollment. Only the Instructor can enroll you in the course, so contact your Instructor.

However, if the Instructor says the course is set to Self Enrollment but no Enroll button shows up, then ask the Instructor to check the Enrollment dates on the course to make sure they are correct. Check to see that other students can enroll as well.