C# The name 'slider' does not exist in the current context

I don’t understand what the problem is … I’ve been trying to solve the problem for 2 days now.
code:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class AudioController : MonoBehaviour
{
    public Sprite audioON;
    public Sprite audioOFF;
    public GameObject buttonAudio;

    public Slider slider_audio;

    private AudioSource audioSr;

    void Start()
    {
        audioSr = GetComponent<AudioSource>();
    }

    void Update()
    {
        audioSr.volume = slider.value;
    }

    public void buttonAudioONOFF()
    {
        if (AudioListener.volume == 1)
        {
            AudioListener.volume = 0;
            buttonAudio.GetComponent<Image>().sprite = audioOFF;
        }
        else
        {
            AudioListener.volume = 1;
            buttonAudio.GetComponent<Image>().sprite = audioON;
        }
    }
}

error:
AudioController.cs(23,26): error CS0103: The name ‘slider’ does not exist in the current context

Your Slider variable name is “slider_audio”, but on line 23 you are referencing the variable as “slider”. On line 23 change “slider” to “slider_audio” (without quotes), and ensure that Inspector shows that variable as being assigned.