Laravel Firebase, eror 404 Not Found saat POST data

Saya menggunakan Laravel 5.4 dan package Kreait/Firebase-php, tapi mengalami error 404 saat mencoba mengirim data.

In Controller :

public function fb(){
        $serviceAccount = ServiceAccount::fromJsonFile(__DIR__.'/FirebaseKey.json');
        $firebase = (new Factory)
            ->withServiceAccount($serviceAccount)
            ->create();
        $database = $firebase->getDatabase();
        $ref = $database->getReference('notifications');
        $key = $ref->push()->getKey();
        $ref->getChild($key)->set([
            'pesan' => 'tes'
        ]);
        return $key;
}

Pesan error :

(1/2) ClientException
Client error: `POST https://myapp-35d0f.firebaseio.com/notifications` resulted in a `404 Not Found` response:
 {
 "error" : "404 Not Found"
 }


in RequestException.php line 113
at RequestException::create(object(Request), object(Response))in Middleware.php line 65
at Middleware::GuzzleHttp\{closure}(object(Response))in Promise.php line 204
at Promise::callHandler(1, object(Response), null)in Promise.php line 153
at Promise::GuzzleHttp\Promise\{closure}()in TaskQueue.php line 48
...........more

(2/2) ApiException
404 Not Found
in ApiException.php line 40
at ApiException::wrapRequestException(object(ClientException))in ApiClient.php line 83
at ApiClient->request('POST', object(Uri), array('json' => null))in ApiClient.php line 61
at ApiClient->push(object(Uri), null)in Reference.php line 353
at Reference->push()in UserController.php line 36
at UserController->fb()
at call_user_func_array(array(object(UserController), 'fb'), array())in Controller.php line 55
......... more

Mohon bantuannya, error tersebut disebabkan karna apa yah?