Call to undefined method App\Notifications\NewUser::via()

So I am following this tutorial

https://dev.to/povilaskorop/laravel-approve-new-registered-users-from-administrator-3nbh

I have followed every step, but when I try to register I got his error

Symfony \ Component \ Debug \ Exception \ FatalThrowableError (E_ERROR)
Call to undefined method App\Notifications\NewUser::via()

Here are my NewUser.php

<?php

namespace App\Notifications;
use App\User;
use Illuminate\Bus\Queueable;
use Illuminate\Notifications\Notification;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Notifications\Messages\MailMessage;
use Illuminate\Notifications\Notifiable;


class NewUser extends Notification
{
    use Queueable;

    private $new_user;

    /**
     * Create a new notification instance.
     *
     * @return void
     */
    public function __construct(User $new_user)
    {
        $this->new_user = $new_user;
    }


    /**
     * Get the mail representation of the notification.
     *
     * @param  mixed  $notifiable
     * @return \Illuminate\Notifications\Messages\MailMessage
     */
    public function toMail($notifiable)
    {
        return (new MailMessage)
            ->line('New user has registered with email ' . $this->new_user->email)
            ->action('Approve user', route('admin.users.approve', $this->new_user->id));
    }
}

And My RegisterController.php

<?php

namespace App\Http\Controllers\Auth;
use App\Notifications\NewUser;
use App\User;
use App\Http\Controllers\Controller;
use Illuminate\Support\Facades\Hash;
use Illuminate\Support\Facades\Validator;
use Illuminate\Foundation\Auth\RegistersUsers;

class RegisterController extends Controller
{
    

    use RegistersUsers;

   
    protected $redirectTo = '/home';

    
    public function __construct()
    {
        $this->middleware('guest');
    }

    
    protected function validator(array $data)
    {
        return Validator::make($data, [
            'name' => ['required', 'string', 'max:255'],
            'email' => ['required', 'string', 'email', 'max:255', 'unique:users'],
            'password' => ['required', 'string', 'min:6', 'confirmed'],
        ]);
    }

   
    protected function create(array $data)
    {
        $user = User::create([
            'name' => $data['name'],
            'email' => $data['email'],
            'password' => Hash::make($data['password']),
        ]);
    
        $admin = User::where('admin', 1)->first();
        if ($admin) {
            $admin->notify(new NewUser($user));
        }
    
        return $user;
    }
}

Any Solutions? I am using the same Laravel version as instructed (Laravel 5.7)

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