DansMaCulotte / laravel-mail-template

src/MailTemplateChannel.php

<?php

namespace DansMaCulotte\MailTemplate;

use DansMaCulotte\MailTemplate\Drivers\Driver;
use Illuminate\Notifications\Notification;

class MailTemplateChannel
{
    /**
     * Send the given notification.
     *
     * @param mixed $notifiable
     * @param Notification $notification
     * @return void
     */
    public function send($notifiable, Notification $notification)
    {
        $message = $notification->toMailTemplate($notifiable);

The method toMailTemplate() does not exist on Illuminate\Notifications\Notification. It seems like you code against a sub-type of Illuminate\Notifications\Notification such as DansMaCulotte\MailTemplate\Tests\WelcomeNotification.

        if ($message instanceof Driver) {
            $message->send();
        }
    }
}