Here are some possible reasons why your course is not in the list of available courses to enroll in.
Too early
Sapling Learning homework typically becomes available two weeks before the first day of class. If your first day of class is more than two weeks away, wait until then and try again.
Too late
Enrollment for Sapling Learning homework typically lasts until the last day of class. If you feel that you should still be able to enroll after the last day of class, please talk to your instructor.
Other
If it's neither too early nor too late for enrollment, your instructor may have asked us to restrict the enrollment dates to a narrower window than described above.
If none of these reasons seem applicable, please contact tech support or talk to your instructor.