hi someone helped me out with this code and when i run it i get this error

1120: Access of undefined property circle_mc.
var blueTween:Tween = new Tween (circle_mc.blue_mc, "alpha", None.easeInOut, 1 , 0 , 1 , true );

im completley new to actionscript and was wondering if someone could tell me what the error means?

Thanks

Code:
import fl.transitions.Tween;
import fl.transitions.TweenEvent;
import fl.transitions.easing.*;
 
 
var yellowTween:Tween = new Tween (circle_mc.yellow_mc, "alpha", None.easeInOut, 1 , 0 , 1 , true );
// this tweens the yellow_mc inside of circle_mc.
 
yellowTween.addEventListener(TweenEvent.MOTION_FINISH, startBlue);
// this listens for the yellowTween to finish its event and when its done it triggers startBlue function.
 
function startBlue(event:TweenEvent):void
{
var blueTween:Tween = new Tween (circle_mc.blue_mc, "alpha", None.easeInOut, 1 , 0 , 1 , true ); 
// in this function it starts the blue_mc to tween. 
 
blueTween.addEventListener(TweenEvent.MOTION_FINISH, startRed);
//also, it listens for the blueTween to finish its motion tween. now, i had to put this inside of this function rather than outside because i declared variable ofblueTween insie of this startBlue function. so in ordor to recognize the blueTween, it has to be within the same location.
 
}
function startRed(event:TweenEvent):void
{
var redTween:Tween = new Tween (circle_mc.red_mc, "alpha", None.easeInOut, 1 , 0 , 1 , true);
// so after blue is finished, it triggered red Tween and this is the red function.
 
}