Facing problem in selecting perfect electric conductor boundaries

Hi
I am trying to design multi mode interference coupler in 3D and I used electromagnetic beam envelope as physics. There in perfect electric conductor option I want to select some boundaries but not able to do so. It is showing overridden or not applicable or nothing is written in the select boundary list. I want to select some other boundaries. Could somebody help me with this problem. Any thing I can read on this problem. Thank You

Sneha Kumari