Courses can be closed to enrollment for several reasons. The most common is that the section is filled to its capacity. In other cases a course may be enrolled "by permission only" because there are special requirements or unusual scheduling options.
The first person to speak to is the instructor for the course. In 99% of cases, the instructor has the final say over course enrollments. If the instructor is not listed for the course, contact the department secretary.
If, after speaking with the course instructor, you still have questions about enrollments, contact the department chair.