Jump Animation 2D problems

I'm new to coding and I'm using Brackeys tutorial and code for 2d player movement and animation. I have all the animations in but I took out the crouch animation in the code and sprites. It's just the jump animation that's not working. I have to double-tap the spacebar to get the jump animation to hold until I touch the ground. If I don't double-tap, then it flashes the jump animation for a frame then sets IsJumping back to false. Otherwise, everything else works out.

Here are the videos

https://www.youtube.com/watch?v=hkaysu1Z-N8

https://www.youtube.com/watch?v=dwcT-Dch0bA!

Here's the code: credit to Brackeys

 using UnityEngine;

 public class PlayerMovement : MonoBehaviour
 {
 public CharacterController2D controller;
 public Animator animator;

 public float runSpeed = 40f;

 float horizontalMove = 0f;

 bool jump = false;


 void Update()
 {

     horizontalMove = Input.GetAxisRaw("Horizontal") * runSpeed;

     animator.SetFloat("Speed", Mathf.Abs(horizontalMove));

     if (Input.GetButtonDown("Jump"))
     {
         jump = true;
         animator.SetBool("IsJumping", true);
     }

 }

 public void OnLanding()
 {
     animator.SetBool("IsJumping", false);
 }

 void FixedUpdate()
 {
     controller.Move(horizontalMove * Time.fixedDeltaTime, jump);
     jump = false;
 }
 }

note: I followed Brackeys' instructions for the transitions and not to transition to self and all the conditions.