There can be several reasons for this:

• The registration deadline for this course expired.
  The deadlines for a course are shown in the course details. You cannot sign up for a course when the deadline has expired. The deadlines are set by the Office for Student Affairs for your degree programme for a good reason. Deadlines affect all students that want to sign up for this course.
• Older module offering
  If you were registered for this module before, a connection to the current course has to be made. This can happen automatically or manually. You won't be able to sign up for the course as long as the connection is not established.
  If you think this has happened to you, please contact the Office for Student Affairs for your degree programme, indicating your matriculation number, name & number of the course, name & number of the module and the semester in which you first registered for the module.
• Individual problem
  In most cases, the problem is an individual one and only single students are affected. In this case, please contact the the Office for Student Affairs for your degree programme, indicating your matriculation number, name & number of the course, name & number of the module and the semester in which you first registered for the module.