I am trying to make character jump in my FPS game. However, he only jumps small height and gets back down to the ground almost immediately. Can anyone help me with the script?

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

[RequireComponent(typeof(CharacterController))]


public class PlayerMovement : MonoBehaviour
{
    private CharacterController _characterController; 

    // Variables for player movement
    public float movementSpeed = 150.0f;
    public float gravity = -9.81f;
    public float jumpForce = 20.0f;
    
    //Ground Check variables
    public Transform groundCheck;
    public float groundDistance = 0.4f; 
    public LayerMask groundMask;

    public bool isGrounded;
    
    void Start()
    {
        _characterController = GetComponent<CharacterController>();

    }
    void Update()
    {
        isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask); //Checks if player is grounded

        if (isGrounded) {

            float deltaX = Input.GetAxisRaw("Horizontal") * movementSpeed;                 // If player is grounded, Get Input keys for moving the player
            float deltaZ = Input.GetAxisRaw("Vertical") * movementSpeed;


            Vector3 movement = new Vector3(deltaX, 0, deltaZ);
            movement = Vector3.ClampMagnitude(movement, movementSpeed);

            movement.y = gravity;                                                         //Sets y component as -9.81

            if (Input.GetButtonDown("Jump1"))                                                //Get Input key for jumping
                {

                movement.y = Mathf.Sqrt(jumpForce * -2.0f * gravity);                    //Calculate movement with physics as ----> Sqrt(h * (-2) *g)
            }

            movement *= Time.deltaTime;
            movement = transform.TransformDirection(movement); 
            _characterController.Move(movement);
        }
       
    }
}

CharacterController.Move only affects the next frame. If you want jump physics, you need a velocity that is stored across frames.

declare:

public float verticalVelocity;

and then change your code to:

if (_characterController.isGrounded && Input.GetButtonDown("Jump1"))
    verticalVelocity = Mathf.Sqrt(jumpForce * -2.0f * gravity);
else if (_characterController.isGrounded)
    verticalVelocity = 0f;
else
     verticalVelocity += gravity * Time.fixedDeltaTime;

movement.y = verticalVelocity;