My jumping script has errors

I have a player movement script in my game and a key aspect of the game is jumping however you cant because my script does not seem to work, I have a screenshot showing what may be a problem and also the script

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

public class PlayerMovement : MonoBehaviour
{
    public CharacterController controller;

    public float speed = 12f;
    public float gravity = -9.81f;
    public float jumpForce = 100f;

    public Transform groundCheck;
    public float groundDistance;
    public LayerMask groundMask;
    public Rigidbody rb;

    Vector3 velocity;
    bool isGrounded;

    // Update is called once per frame
    void Update()
    {

        isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);

        if (isGrounded && velocity.y < 0)
        {
            velocity.y = -2f;
        }
        float x = Input.GetAxis("Horizontal");
        float z = Input.GetAxis("Vertical");

        Vector3 move = transform.right * x + transform.forward * z;

        controller.Move(move * speed * Time.deltaTime);

        velocity.y += gravity * Time.deltaTime;

        controller.Move(velocity * Time.deltaTime);
 
        if (Input.GetKeyDown("space"))

        {
            rb.AddForce(0, jumpForce, 0, ForceMode.Impulse);
        }
    }
}

I think this might be the issue but I’m not sure

if (isGrounded && velocity.y < 0)
         {
             velocity.y = -2f;
         }

also there is already gravity implemented in unity, unless you set that to 0 then you have double gravity, but ya I implemented gravity like that in my current project, but you should do it like this:

if(!grounded)
    velocity.y += gravity * Time.deltaTime;