Edit: resolved. I just downgraded to slim 3.
I'm getting the following error message. I already tried "composer require slim/http" and "composer require slim/psr7".
Uncaught RuntimeException: Could not detect any PSR-17 ResponseFactory implementations. Please install a supported implementation in order to use AppFactory::create(). See https://github.com/slimphp/Slim/blob/4.x/README.md for a list of supported implementations. in /Applications/XAMPP/xamppfiles/htdocs/MyApi/vendor/slim/slim/Slim/Factory/AppFactory.php:166 Stack trace: #0 /Applications/XAMPP/xamppfiles/htdocs/MyApi/vendor/slim/slim/Slim/Factory/AppFactory.php(92): Slim\Factory\AppFactory::determineResponseFactory() #1 /Applications/XAMPP/xamppfiles/htdocs/MyApi/public/index.php(12): Slim\Factory\AppFactory::create() #2 {main} thrown in /Applications/XAMPP/xamppfiles/htdocs/MyApi/vendor/slim/slim/Slim/Factory/AppFactory.php on line 166