I am unable to use selection set Command in 3DX CATIA Assy Design.

Hello Everyone, i am unable to use the selection set command in 3DX CATIA Assy Design.first of all its wont be coming in action bar==> Tools,next i have added this command to action bar where it remains grade-out .

if anyone knows please help me.

Thanks in Advance.

​​​​​​​