Movement Script in C# for Unity

Move.cs

private float speed = 2.0f;
public GameObject character;

void Update () {
		
		if (Input.GetKey(KeyCode.RightArrow)){
			transform.position += Vector3.right * speed * Time.deltaTime;
		}
		if (Input.GetKey(KeyCode.LeftArrow)){
			transform.position += Vector3.left* speed * Time.deltaTime;
		}
		if (Input.GetKey(KeyCode.UpArrow)){
			transform.position += Vector3.forward * speed * Time.deltaTime;
		}
		if (Input.GetKey(KeyCode.DownArrow)){
			transform.position += Vector3.back* speed * Time.deltaTime;
		}
	}

making rigidbody a class in unity doesn't work for me. it gives me an error