How to fix player jumping while watching downwards and moving with s key ?

Thanks for you reply. Please give me some time and I will update accordingly.

Thanks it worked!. But still whenever I look very top and to very bottom and move the player speed decreases.

I didn't make it very detailed but I write a simple fps player controller script, you can add your own camera controls to camera related lines, and the camera must be the child of the moving object.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class player : MonoBehaviour
 {
     private CharacterController controller;
     private Vector3 playerVelocity;
     private bool groundedPlayer;
     public float playerSpeed = 2.0f;
     private float jumpHeight = 1.0f;
     private float gravityValue = -9.81f;
     public Transform Camera;
     public float sens = 2f;
     // Start is called before the first frame update
     void Start()
     {
         controller = GetComponent<CharacterController>();
     }
 
     // Update is called once per frame
     void Update()
     {
         groundedPlayer = controller.isGrounded;
         if (groundedPlayer && playerVelocity.y < 0)
         {
             playerVelocity.y = 0f;
         }
 
         // Debug.Log(transform.forward);
         Vector3 forward = transform.forward * Input.GetAxis("Vertical");
         Vector3 sideward = transform.right * Input.GetAxis("Horizontal");
         Vector3 move = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
         controller.Move((forward + sideward) * Time.deltaTime * playerSpeed);
         
         Vector3 cameraHorizontalAxis = Camera.localEulerAngles;
         Vector3 cameraVerticalAxis = transform.localEulerAngles;
         cameraHorizontalAxis.x -= Input.GetAxis("Mouse Y") * sens;
         cameraVerticalAxis.y += Input.GetAxis("Mouse X") * sens;
         transform.localRotation = Quaternion.AngleAxis(cameraVerticalAxis.y, Vector3.up);
         Camera.localRotation = Quaternion.AngleAxis(cameraHorizontalAxis.x, Vector3.right);
 
 
         // Changes the height position of the player..
         if (Input.GetButtonDown("Jump") && groundedPlayer)
         {
             playerVelocity.y += Mathf.Sqrt(jumpHeight * -3.0f * gravityValue);
         }
 
         playerVelocity.y += gravityValue * Time.deltaTime;
         controller.Move(playerVelocity * Time.deltaTime);
     }
 }