How to rotate camera and player C#

Hi there, I want to move my camera and my player simultaneously within the same script using EulerAngles, however if I assign transforms like so: "Transform player = GameObject.FindGameObjectWithTag("Player").transform;" and then i call it with eulerangles like so: "player.eulerAngles = player.eulerAngles - (rotation * sensitivity);". Visual studios then tells me that "The name player does not exist in current context", how do i fix this?
Full code:

Code (CSharp):

1. public class CameraController : MonoBehaviour
2. {
3.     public float sensitivity = 2.0f;
4.     public float X;
5.     public float Y;
6.     public Vector3 rotation;
7.  
8.     void Start()
9.     {
10.         Transform player = GameObject.FindGameObjectWithTag("Player").transform;
11.         Transform mainCamera = GameObject.FindGameObjectWithTag("MainCamera").transform;
12.     }
13.  
14.     void Update()
15.     {
16.         X = Input.GetAxis("Mouse Y");
17.         Y = Input.GetAxis("Mouse X");
18.  
19.         Vector3 rotation = new Vector3 (X, -Y, 0);
20.         transform.eulerAngles = transform.eulerAngles - (rotation * sensitivity);
21.         player.eulerAngles = player.eulerAngles - (rotation * sensitivity);
22.     }
23. }

**X and Y are flipped because for some reason if they are set normally, moving the mouse left and right rotates the camera up and down.

 

Line 10 makes a local variable, as does Line 11.

Nobody outside of Start() can see those variables.

Instead, make a class variable (like your top variables), then do the find operations in Start(), removing the type declaration from those lines.