Warning (512) : Unable to emit headers. Headers sent in file=/var/www/html/src/View/Helper/UtilHelper.php line=86 [in /var/www/html/vendor/cakephp/cakephp/src/Http/ResponseEmitter.php, line 71]
Warning (2) : Cannot modify header information - headers already sent by (output started at /var/www/html/src/View/Helper/UtilHelper.php:86) [in /var/www/html/vendor/cakephp/cakephp/src/Http/ResponseEmitter.php, line 168]
Warning (2) : Cannot modify header information - headers already sent by (output started at /var/www/html/src/View/Helper/UtilHelper.php:86) [in /var/www/html/vendor/cakephp/cakephp/src/Http/ResponseEmitter.php, line 197]
Error: Missing Controller

Missing Controller 📋

Cake\Http\Exception\MissingControllerException
Toggle Vendor Stack Frames

Error Sitemap.xmlController could not be found.

            strpos($controller, '/') !== false ||
            strpos($controller, '.') !== false ||
            $firstChar === strtolower($firstChar)
        ) {
            throw $this->missingController($request);
        }
        /** @var class-string<\Cake\Controller\Controller>|null */
        return App::className($pluginPath . $controller, $namespace, 'Controller');

In the case you tried to access a plugin controller make sure you added it to your composer file or you use the autoload option for the plugin.

Suggestion Create the class Sitemap.xmlController below in file: src/Controller/Sitemap.xmlController.php

    <?php
    
namespace App\Controller;

    use 
App\Controller\AppController;

    class 
Sitemap.xmlController extends AppController
    
{

    }

If you want to customize this error message, create templates/Error/missing_controller.php