This is usually caused by the Course being a 2 Term (or full year) Course, and being in the Second Term (or any term other than the first). To fix this, you will need to toggle back to the First Term to be able add a Section to a Course.

To do this, navigate to:
  1. onCampus > Procedure > Academics > Course Sections, and filter for the School Level and First Term 
  2. Expand the Department the Course is in
  3. Click Manage Sections to create sections for the course
  4. Click + Add Section