A sphere is unable to roll on

For pure rolling, the velocity of the point of contact with the surface must be zero.

On a smooth inclined surface, the component of gravity parallel to the incline causes a linear acceleration $a = g \sin \theta$.

Since the surface is smooth, there is no frictional force. The normal reaction and gravitational force both pass through the center of mass, so the net torque about the center of mass is zero.

Because the torque is zero, the angular acceleration $\alpha$ is zero. The angular velocity $\omega$ remains constant while the linear velocity $v$ increases continuously.

Thus, the condition for pure rolling $v = \omega R$ cannot be maintained, and the sphere will slip.

On a smooth horizontal surface, a sphere can roll if initially set with $v = \omega R$, as both $v$ and $\omega$ remain constant. On rough surfaces, friction provides the necessary torque to maintain or achieve rolling.