Question

A car is moving with uniform velocity on a rough horizontal road.Then according to Newton's first law of motion

Answer 1

an external force is required to stop the car state of uniform motion

Answer 2

Rough road will provide frictional force in the direction opposite to the direction of motion. Frictional force will be external force.
So, according to Newton's first law, the car will change its state of motion to rest because of an external force (friction) unless any other external force in the opposite direction to the friction and more in magnitude than friction acts on it.

Thanks.