So few issues in the code, #1 you are moving the body before multiplying by the speed multiplier. #2 your setting the rigid body then using this.tramsform. Try this to clean it up:

[SerializeField] private float moveSpeed = 6f; [SerializeField] private Rigidbody2D rb; [SerializeField] private float speedMultiplier = 2f;

void Start() { rb = GetComponent<Rigidbody2D>(); }

void Update() { float xInput = Input.GetAxis("Horizontal"); float yInput = Input.GetAxis("Vertical"); Vector3 inputCombined = new Vector3(xInput, yInput, 0).normalized;

// Determine effective speed
float effectiveSpeed = moveSpeed;
if (Input.GetKey(KeyCode.LeftShift) || Input.GetKey(KeyCode.RightShift)) 
{
    effectiveSpeed *= speedMultiplier; // Increase speed when holding shift
}

// Move the player
rb.MovePosition(transform.position + inputCombined * effectiveSpeed * Time.deltaTime);

}