The character moves but does not jump. What is the problem?

Question

Unity

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

public class Player : MonoBehaviour
{
    Rigidbody2D rb;
    public float speed;
    public float jumpHeight = 13.0f;

    void Start()
    {
        rb = GetComponent<Rigidbody2D>();
    }
    
    void Update()
    {
        Flip();
    }

    void FixedUpdate()
    {
        rb.velocity = new Vector2(Input.GetAxis("Horizontal")* speed, rb.velocity.y);
        if (Input.GetKeyDown(KeyCode.Space))
        {
            rb.AddForce(transform.up * jumpHeight, ForceMode2D.Impulse);
            
        }
    }

    void Flip()
    {
        if (Input.GetAxis("Horizontal") > 0)
        {
            transform.localRotation = Quaternion.Euler(0, 0, 0);
        }
        if (Input.GetAxis("Horizontal") < 0)
        {
            transform.localRotation = Quaternion.Euler(0, 180, 0);
        }

    }
}