Figure shows two blocks P & Q kept in contact with the wall with the force F. The wall is smooth but the surfaces in between the blocks are rough, for the block to be at rest against the wall P is

Since the combined weight of P and Q is vertically downward , to keep the system in equilibrium , a vertically upward force must be acting on it. Friction between P and the wall can only supply this force. Since the wall is frictionless , the system cannot be in equilibrium.