Classes in different units are often scheduled at the same time. When you have two units with classes at the same time, you'll see a clash on CAS. 

1. Can you fix it? 
Can you select another class time for one of the two classes that are clashing?

To fix this: log in to CAS and select a new class time. You can edit your allocations up until approximately 2 weeks into semester, check the CAS website for specific dates.

Is one of the clashing classes a lecture?

To fix: allocate yourself to the online lecture. These recordings exist so you can make it to the classes you need to.

Is there a full class you can waitlist for?

To fix: Waitlist for the full class, you will be allocated if a space becomes available.

2. You can't fix it
If you can't do any of the above, submit an online request.