Check the Course Catalog to see if you meet all of the prerequisites.
If a course has certain prerequisites that you have not completed, or if the course is restricted to only students in certain program or graduate-level students, email the professor. They may be able to manually register you for the course, or suggest an alternative.
As courses change each semester, not all courses are always being offered. Some courses are offered in alternating years and some may no longer be offered. See the Course Catalog for up-to-date offerings.
Certain courses may be listed as Special Topics. You can view Special Topics offerings in the Course Catalog. If you do not possess the prerequisites or if it is a 500-level course or above and MyPack says it is restricted to graduate students, email the instructor to see if they will register you for the course. When registering, double-check the section of the Special Topics course because each Special Topics section is a completely different course.