I want to enable PlayerPrefs to the text so it display the last silder value on Start

Unity Engine
1 
 using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using TMPro;
public class ShowValue : MonoBehaviour
{
     [SerializeField] private Slider _slider;
     [SerializeField] private TextMeshProUGUI _sliderText;
     void Start()
     {
         _slider.onValueChanged.AddListener((v) =>{
             _sliderText.text = v.ToString("0");
         });
     }
}
2

How to report your problem productively in the Unity3D forums:

http://plbm.com/?p=220

How to understand compiler and other errors and even fix them yourself:

Here’s an example of simple persistent loading/saving values using PlayerPrefs:

Useful for a relatively small number of simple values.