Moving an object using rigidbody.AddForce & keyboard input.

I'm making a simple top down shooter to get to grips with Unity and have $$anonymous$$t a snag.

When the player moves wit$$anonymous$$n range of an enemy, the enemy should start following the player. T$$anonymous$$s works using the following in FixedUpdate:

 function FixedUpdate()
 {
 
 if (player)
     {
          distance = Vector3.Distance( player.transform.position, transform.position);
         
         if ( distance < 1.5 && distance > 0.05 ) 
         {
             var delta = player.transform.position - transform.position;
             delta.Normalize();
     
             var speed = moveSpeed * Time.deltaTime;
             print("close");
           
             rigidbody.MovePosition(rigidbody.position + (delta * speed));
 
         }
     }
 
 }

However, It doesn't work if I use:

 rigidbody.AddForce(rigidbody.position + (delta * speed));

Furthermore, it doesn't even work if I use:

 rigidbody.AddForce(transform.forward * 10000,ForceMode.Impulse);

There's just no movement at all when using AddForce. It's probably somet$$anonymous$$ng really simple but I couldn't find an answer on the forums/online.

Has anyone encountered t$$anonymous$$s before?