You can find more information here: https://www.wiwiss.fu-berlin.de/studium-lehre/pruefungsbuero/cm-faq/index.html#faq_cm-teilanmeldung
First check your course history in Campus Management. Since you probably attend many courses, you may have missed something.
Otherwise, you should check with your responsible examination office
Check whether you have also set your preference.
Possibly your participation was not recorded correctly. Please contact your responsible examination office.