There may be a number of reasons why you can't enter preferences in CAS:

1. CAS may not be open for entering preferences: You can only enter preferences at specific times of the year. Check the CAS website for relevant dates.
2. Not all units use CAS for entering preferences or allocating students to class times, or the unit coordinator may have disabled it for a specific reason. Further information will be provided by CAS in this case.
3. Your unit is a Summer School or non-standard unit.  After enrolling you will be automatically allocated class times within 24 hours. Once allocated you may then make changes to your classes subject to availability. 
4. You can get into the unit in CAS but there aren't any options available because there are no remaining spaces: Waitlist yourself for a class time that works for you.