My spaceship does not move forward when mouse is pressed down

To summarize what I want it to do: I am trying to code it so that when one button is pressed, the player's ship will start moving forward in the direction that it is facing. If a different button is pressed, the player's ship will start moving backward from the direction that it is facing.
I starting coding it with rigidbody2d, but it doesn't move forward or backwards when I press them down. Tried many times to fix it, to figure out where I am going wrong, but I cannot find out what code is being problematic. Here is the code right now:

 public class SpaceShip : MonoBehaviour
 {
     public float acceleration;
     private Rigidbody2D rb2d;
 
     private Vector2 forward;
 
     void Start()
     {
         rb2d = GetComponent<Rigidbody2D>();
     }
 
     private void FixedUpdate()
     {
         forward = transform.position + transform.forward;
         if (Input.GetMouseButton(0))
         {
             rb2d.AddForce(forward * acceleration);
         }
         else if (Input.GetMouseButton(1))
         {
             rb2d.AddForce(-forward * acceleration);
         }
     }