The archived courses are not enrollable. They are courses that are in storage that have run in past years. They are not maintained after archiving.
Many of these have current copies that are running though.
The only courses enrolling at a given time will enroll from the home page of the site.
AYOP (At Your Own Pace) courses in math and science enroll all year long with no enrollment limits.
Live classes only enroll during the summer (starting around May as we get volunteers to August 4). Live classes can only seat a limited amount of students in each class. They are subject to lottery enrollment if we get more students wanting to take the class than volunteer teachers and graders stepping up.
See this section of our Knowledge Base for information about enrollment and lottery seating: