phpでフォーム処理のスクリプトを作りたい

前提・実現したいこと

phpとslimを使用してフォーム処理のスクリプトを作りたいんですが、入力フィールドに値を入力するとエラーが出ます。

発生している問題・エラーメッセージ

Fatal error: Uncaught Slim\Exception\HttpMethodNotAllowedException: Method not allowed. Must be one of: GET in C:\Users\Santa\my_slim\vendor\slim\slim\Slim\Middleware\RoutingMiddleware.php:96 Stack trace: #0 C:\Users\Santa\my_slim\vendor\slim\slim\Slim\Routing\RouteRunner.php(72): Slim\Middleware\RoutingMiddleware->performRouting(Object(Slim\Psr7\Request)) #1 C:\Users\Santa\my_slim\vendor\slim\twig-view\src\TwigMiddleware.php(125): Slim\Routing\RouteRunner->handle(Object(Slim\Psr7\Request)) #2 C:\Users\Santa\my_slim\vendor\slim\slim\Slim\MiddlewareDispatcher.php(140): Slim\Views\TwigMiddleware->process(Object(Slim\Psr7\Request), Object(Slim\Routing\RouteRunner)) #3 C:\Users\Santa\my_slim\vendor\slim\slim\Slim\MiddlewareDispatcher.php(81): class@anonymous->handle(Object(Slim\Psr7\Request)) #4 C:\Users\Santa\my_slim\vendor\slim\slim\Slim\App.php(215): Slim\MiddlewareDispatcher->handle(Object(Slim\Psr7\Request)) #5 C:\Users\Santa\my_slim\vendor\slim\slim\Slim\App.php(199): Slim\App->handle(Object(Slim\Psr7\Request)) #6 C:\Us in C:\Users\Santa\my_slim\vendor\slim\slim\Slim\Middleware\RoutingMiddleware.php on line 96
```


### 該当のソースコード

```php
<?php 
use \Psr\Http\Message\ServerRequestInterface as Request;
use \Psr\Http\Message\ResponseInterface as Response;
use \DI\Container;
use \Slim\Views\Twig;
use \Slim\Views\TwigExtension;
use \Slim\Views\TwigMiddleware;
use \Slim\Factory\AppFactory;
use \Slim\Middleware\RoutingMiddleware;

require __DIR__ . '/../vendor/autoload.php';

// DI Container

$container =new Container();

//create App
AppFactory::setContainer($container);
$app = AppFactory::create();

//set view in container
$container->set('view',function(){
    return Twig::create('./templates',['cache'=>'./templates/cache']);
});
// Add Twig-View Middleware
$app->add(TwigMiddleware::createFromContainer($app));


//'/'route

$app->get('/',function ($request, $response) {
    return $this->get('view')->render($response,'index.html',[
        'message' => 'please type...',
        'class' =>' ',
    ]);
});

$app->post (' /',function ($request, $response) {
    $body = $request->getParsedBody();
    $msg  = $body['msg'];
    return $this->get('view')->render($response,'index.html', [
        'message' => 'you typed:' . $msg . '.',
        'msg' => $msg,
        'class' => 'alert alert-primary',
    ]);
});

$app->run();

```
```html
<!DOCTYPE html>
<html lang="ja">
    <head>
          <meta charset="utf-8">
          <title>Index</title>
          <link rel="stylesheet"
            href = "https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.css">
    </head>
    <body class="container">
        <h1 class="display-4">Index</h1>
        <p class="{{class}}">{{ message }}</p>
        <form action="/" method="post">
        <input type="text" name="msg" value="{{msg}}">
        <input type="submit">
    </form>
    </body>
</html>
```

### 試したこと

templatesの場所を変えたりしたのですがあまり分かりませんでした。
### 補足情報（FW/ツールのバージョンなど）

ここにより詳細な情報を記載してください。