Abaqus Symmetry along crack

Hi
 

 I used symmetry through the crack line. when I tried to assign seem to the crack (edge of geometry) 
 I got a warning that the seem cannot be defined on exterior edges. I know that when I will define the crack later I will choose it is on the symmetric plane (half-model). such option in Abaqus implies that the crack on the symmetric line can be modeled, isn't it?
 

 What am I missing here, please?