AddForce is not working

Can anyone help me fix the bug? I put a script for my player tank and it has no problem shooting, but when I try to put similar another script to other tank as the "AI" is not working?

This is the code for the player tank:

 var prefabBullet : Rigidbody;
 
 var shootForce : float;
 
 var shootPosition : Transform;
 
 var fireRate = 1.0;
 
 private var nextFire = 0.0;
 
 function Update () {
 
     if(Input.GetKeyDown("space") && Time.time > nextFire) {
         nextFire = Time.time + fireRate;
         var instanceBullet = Instantiate(prefabBullet, transform.position,shootPosition.rotation);
         instanceBullet.rigidbody.AddForce(shootPosition.forward * shootForce);
     }
 }
 
 
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This one is for the AI tank:

 var prefabBulletAI : Rigidbody;
 
 var shootForceAI : float;
 
 var shootPositionAI : Transform;
 
 var fireRate = 1.0;
 
 private var nextFire = 0.0;
 
 function Update () {
 
     if (Input.GetKey("up") && Time.time > nextFire) {
         nextFire = Time.time + fireRate;
 
         var instanceBulletAI = Instantiate(prefabBulletAI, transform.position,shootPositionAI.rotation);
         instanceBulletAI.rigidbody.AddForce(transform.forward * shootForceAI);
     }
 }

The problem is why the AI bullet is not moving forward, when there is no error. The bullet simply doesn't have any force that move it forward eventhough the script for player is working properly. Please help me fix this bug. Thank you