I need help with grounding.

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

public class PlayerControls : MonoBehaviour
{
    public Rigidbody2D rb;
    public Transform groundCheck;
    public float groundCheckRadius;
    public LayerMask whatIsGround;
    private bool onGround;




    void FixedUpdate()
    {
        onGround = Physics2D.OverlapCircle(groundCheck.position, groundCheckRadius, whatIsGround);
    }
    void Start()
    {
       
    }
    void Update()
    {
      
        rb.velocity = new Vector2(5, rb.velocity.y);
       

        ; if (Input.GetMouseButtonDown(0)
        ){

            rb.velocity = new Vector2(rb.velocity.x, 7);
        }




    }
}

What have i done wrong here?