Twilio WhatsApp Integration Not Working

Hello folks, I am now trying to send WhatsApp notifications to the tenants when the transaction is created. I have followed as Twilio guides in this tutorial, https://www.twilio.com/blog/create-laravel-php-notification-channel-whatsapp-twilio but it returns 'null' when I try to fire the function as it described.

I have mentioned my codes below. Please check them and help me with this. Btw, this is the last function I have to implement in my current project.

App\Channels\WhatsAppChannel.php

<?php
namespace App\Channels;

use Twilio\Rest\Client;
use Illuminate\Notifications\Notification;


class WhatsAppChannel
{
    public function send($notifiable, Notification $notification)
    {
        $message = $notification->toWhatsApp($notifiable);


        $to = $notifiable->routeNotificationFor('WhatsApp');
        $from = config('services.twilio.whatsapp_from');


        $twilio = new Client(config('services.twilio.sid'), config('services.twilio.token'));


        return $twilio->messages->create('whatsapp:' . $to, [
            "from" => 'whatsapp:' . $from,
            "body" => $message->content
        ]);
    }
}

App\Channels\Messages\WhatsAppMessage.php

<?php

namespace App\Channels\Messages;

class WhatsAppMessage
{
  public $content;

  public function content($content)
  {
    $this->content = $content;

    return $this;
  }
}

App\Tenant.php

// Twilio
    public function routeNotificationForWhatsApp() {
        return $this->phone_no;
    }

App\Notifications\InvoiceProcessed.php

<?php

namespace App\Notifications;

use App\Models\Transaction;
use Illuminate\Bus\Queueable;
use App\Channels\WhatsAppChannel;
use App\Channels\Messages\WhatsAppMessage;
use Illuminate\Notifications\Notification;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Notifications\Messages\MailMessage;

class InvoiceProcessed extends Notification
{
    use Queueable;

    public $transaction;

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

    /**
     * Get the notification's delivery channels.
     *
     * @param  mixed  $notifiable
     * @return array
     */
    public function via($notifiable)
    {
        return [WhatsAppChannel::class];
    }

    /**
     * Get the mail representation of the notification.
     *
     * @param  mixed  $notifiable
     * @return \Illuminate\Notifications\Messages\MailMessage
     */
    public function toWhatsApp($notifiable)
  {
        // $orderUrl = url("/orders/{$this->order->id}");
        // $company = 'Acme';
        // $deliveryDate = $this->order->created_at->addDays(4)->toFormattedDateString();
        $tenant_name = $this->transaction->tenant->name;
        $start_date = $this->transaction->start_date;
        $end_date = $this->transaction->end_date;
        $total_price = $this->transaction->total_price;

        $content = $tenant_name . ' has rented bedspace(s) from ' . $start_date . ' to ' . $end_date . ' at the fees of ' . $total_price;


        return (new WhatsAppMessage)->content($content);
  }

    /**
     * Get the array representation of the notification.
     *
     * @param  mixed  $notifiable
     * @return array
     */
    public function toArray($notifiable)
    {
        return [
            //
        ];
    }
}

App\Http\Controllers\Operations\TransactionController.php (Store Method)

// There are codes for storing the transaction, updating the tenant with the transaction id, and other required methods here

$tenant->notify(new InvoiceProcessed($transaction));