How to Ground check a character?

I am following a tutorial and I don't know if it's because I have the newer version or I did something incorrectly. (I realized some of the scripts were not valid in the updated Unity.) I can constantly hit the space bar and my character will continue to jump. I tried making an 'Empty Child' but it didn't recognize that I placed with the player. I have no clue how to fix this issue.

 using UnityEngine;
 using System.Collections;
 
 public class Rose_Contoller : MonoBehaviour {
     
 
     public Transform groundCheck;
     public float groundCheckRadius;
     public LayerMask WhatIsGround;
     private bool grounded;
     
     public float moveSpeed;
     public float jumpHeight;
     
     // Use this for initialization
     void Start () {
     }
     void FixedUpdate () {
         grounded = Physics2D.OverlapCircle(groundCheck.position,groundCheckRadius,WhatIsGround);
     } 
     // Update is called once per frame
     void Update () {
         
         if (Input.GetKeyDown(KeyCode.Space)){
             
             GetComponent <Rigidbody2D>().velocity = new Vector2(GetComponent<Rigidbody2D>().velocity.x,jumpHeight);
         }
         
         if (Input.GetKey(KeyCode.D)){
             
             GetComponent <Rigidbody2D>().velocity = new Vector2(moveSpeed,GetComponent<Rigidbody2D>().velocity.y);
         }
         
         if (Input.GetKey(KeyCode.A)){
             
             GetComponent <Rigidbody2D>().velocity = new Vector2(-moveSpeed,0);
         }
     }
 }