buckling exclude of an area in Optistruct optimization

Dear support team,

I tryed to start an optimization with two Load steps. The first is a general static load step and the second is a buckling loads as subcase which I need for my buckling response. 

The problem is that I want to Exclude the area of the rib in the buckling Analysis, but it can't find the Panel-Set. Where is the failure ?

Unable to find an attachment - read this blog