The following code produces the error:
if (hitInfo.transform.name == "jumppad" && isJumping == false)
The error is being given because the raycast is not hitting anything such as a collider, it is just going out into open space.
This is the error that is showing up:
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
public class gun_script : MonoBehaviour
{
    public Transform firepoint;
    public LineRenderer linerend;
    public Transform gun;
    public float mouseSens;
    public int jumpHeight = 5;
    float lookangle;
    public Rigidbody2D rbForPlayer;
    Vector2 lookDirection;
    private bool isJumping = false;
    // Update is called once per frame
    void Update()
    {
        isJumping = false;
        lookDirection = Camera.main.WorldToScreenPoint(Input.mousePosition);
        lookangle = Mathf.Atan2(lookDirection.y, lookDirection.x) * Mathf.Rad2Deg;
        gun.rotation = Quaternion.Euler(0, 0, lookangle * mouseSens);
        RaycastHit2D hitInfo = Physics2D.Raycast(firepoint.position, firepoint.right);
        if (hitInfo)
        {
            linerend.SetPosition(0, firepoint.position);
            linerend.SetPosition(1, hitInfo.point);
        }
        else
        {
            linerend.SetPosition(0, firepoint.position);
            linerend.SetPosition(1, firepoint.position + firepoint.right * 100);
        }
        if (hitInfo.transform.name == "jumppad" && isJumping == false)
        {
            Invoke("Jump", 1f);
        }
    }
    void Jump()
    {
        isJumping = true;
        rbForPlayer.AddForce(new Vector2(0, jumpHeight));
    }
}
 
    