Newton's First Law of Motion applies to:

Newton's First Law states that a body remains at rest or in uniform motion in a straight line unless and until an unbalanced force acts on it.

The law of inertia is another name for Newton's First Law.

Inertia is the property of a body's inability to change its resting position or uniform motion in a straight line.

Newton's first law of motion describes the state of rest or uniform motion and is only applicable in the Inertial Frame.

Newton's first law is always incorrect in accelerating frames because an object at rest in an inertial frame can appear to accelerate in a non-inertial frame despite no force acting on it.

Assume a man is standing in a truck that is moving at a high rate of acceleration.

Another person is standing motionless on the ground.

Case 1: The person standing on the ground observes the truck moving forward with a man standing still on it.

Case 2: The man in the truck witnesses the person on the ground moving backwards with an acceleration 'a' where no force is visible.

Because this is incorrect, Newton's laws are invalid in a non-inertial frame.