Rotation help wanted

Unity Engine
1

I’m trying to make an NPC face my character whenever they get close enough to each other. The problem is that I tried what I thought work (transform.rotation -= player.transform.rotation) but it threw a error that basically said Quaternions can’t work with simple math operands. Does anyone have a solution?

2 
using UnityEngine;

public class Example : MonoBehaviour
{
    // The object whose rotation we want to match.
    Transform target;

    // Angular speed in degrees per sec.
    float speed;

    void Update()
    {
        // The step size is equal to speed times frame time.
        var step = speed * Time.deltaTime;

        // Rotate our transform a step closer to the target's.
        transform.rotation = Quaternion.RotateTowards(transform.rotation, target.rotation, step);
    }
}