adjust the player in the ground And movements

 //C#
 void OnCollisionStay (Collision col) {
     if (col.collider.gameObject.tag == "Ground") { //Check the collider is ground
         transform.up = col.contacts[0].normal;
     }
 }
 
 //JS
 function OnCollisionStay (col : Collision) {
     if (col.collider.gameObject.tag == "Ground") { //Check the collider is ground
         transform.up = col.contacts[0].normal;
     }
 }

Ok. I made a short example of what you are trying to achieve. What you need is a ground with some 2D collider and a Player object with this script only. I still don't have enough time to polish it, but you will get the picture:

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


 public class Player : MonoBehaviour
 {
 private void Update()
 {
     RaycastHit2D hit = Physics2D.Raycast(transform.position, Vector3.down, Mathf.Infinity);

     // Get the perpendicular normal
     Vector3 direction = new Vector3(hit.normal.y * -1, hit.normal.x);
     // Move right
     if (Input.GetKey(KeyCode.D))
     {
         transform.position += direction * -1 * Time.deltaTime;
         transform.localScale = new Vector3(1, transform.localScale.y, transform.localScale.z);
         transform.rotation = Quaternion.LookRotation(Vector3.forward, hit.normal);
     }
     // Move left
     else if (Input.GetKey(KeyCode.A))
     {
         transform.position += direction * Time.deltaTime;
         transform.localScale = new Vector3(-1, transform.localScale.y, transform.localScale.z);
         transform.rotation = Quaternion.LookRotation(Vector3.forward, hit.normal);
     }
 }
 }

This example does not rely on physics therefore you need to implement your own rules for gravity and other stuff, but the result can be observed when you hit play. In real good platformer kind of game it's good to set your own rules even if it's harder at first.