Problem with enemy Knockback.

You've got a couple problems here -

1. Using a while loop like this will not apply a force over time. The while loop will run all the way through every frame, so the end result will be a large amount of force being applied all at once. I could help you reorganize your code to successfully apply force over time, but I think there is a better solution for you. **

2. You are setting your velocity every frame while also trying to add a force. The result of this is that the force is immediately canceled out. If you apply a force during one frame, then the physics system will calculate a change in velocity and apply it to the rigidbody. But if you then set the velocity the next frame, you are effectively overwriting whatever velocity change the physics system just calculated for you.

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For damage knockback I recommend keeping track of this yourself rather than handing it over to the physics system. The code is simple - just keep track of your own velocity vector due to outside forces and add it to your rigidbody velocity every frame. Then you can scale this vector down over time. Here is the basic code, but you will have to fit this in to your own code

 public class MyCharacterController : Monobehavior{
     private Vector3 outsideForces;
 
     //You can adjust this
     public float forceDecayRate = 1f;
 
     public void AddForce(Vector3 force){
         outsideForces += force;
     }
 
     void Update(){
         //blah blah blah
         Move();
 
         //Decay force over time
         outsideForces = Vector3.Lerp(outsideForces, Vector3.zero, forceDecayRate*Time.deltaTime);
     }
 
     private void Move(){
         //....
 
         Vector2 newVelocity = Vector2.zero;
 
         if(targetPosition.x > transform.position.x){
             newVelocity = new Vector2(speed, rb.velocity.y);
         }
         else if(targetPosition.x <>> transform.position.x){
             newVelocity = new Vector2(-speed, rb.velocity.y);
         }
 
         rb.velocity = newVelocity + outsideForces;
     }
 }

When you need to apply knockback, just call the AddForce() method in your controller script with the desired force vector. You can even use this to apply multiple knockbacks at the same time.