How to make a smooth rotation?

[SarperS]

You are trying to lerp to the same position you are already in. Try this and let me know if there is anything you need explained.

private float desiredRot;
	public float rotSpeed = 250;
	public float damping = 10;

	private void OnEnable() {
		desiredRot = transform.eulerAngles.z;
	}

	private void Update() {
		if (Input.GetMouseButton(0)) {
		     if (Input.mousePosition.x > Screen.width / 2) desiredRot -= rotSpeed * Time.deltaTime;
		     else desiredRot += rotSpeed * Time.deltaTime;
		 }

		var desiredRotQ = Quaternion.Euler(transform.eulerAngles.x, transform.eulerAngles.y, desiredRot);
		transform.rotation = Quaternion.Lerp(transform.rotation, desiredRotQ, Time.deltaTime * damping);
    }