thin surface
 

i want to model duct fan and I have created two parts one cylinder is fan inside a bigger cylinder I like to put a thin wall on the cylindrical surfaces of the smaller cylinder to define it as a duct fan.

This is the error i get please help thanks

Could not create the thin surface partner for boundary 'Boundary 1'. Only the specified wall boundary will be created.The primitive region 'F27.6' does not possess a corresponding internal surface.