How to use external api to print data via shortcode

Hi, I’m new to php and tries to print out an external api to the wordpress-page with shortcode in the plugin that I created.
But the code I have is not working, what is wrong? Here is the code:

add_shortcode('external_data', 'callback_function_name');

function callback_function_name() {
    
    $url = 'https://api.fortnox.se/3/articles/';
    $args = array(
        'headers' => array(
          'Content-Type' => 'application/json',
          'Accept' => 'application/json',
          'Access-Token' => 'accessToken',
          'Client-Secret' => 'ClientSecret'
        )
        );

        $response = wp_remote_get( $url, $args );

        if (is_wp_error($response)) {
            $error_message = $response->get_error_message();
            return "Something went wrong: $error_message";
        }

        $results = json_decode(wp_remote_retrieve_body($response));

    
        foreach($results as $result) {
        $html += "<p>" . $result->Articles->Description . "</p>";
        }

        return $html;