Show username only if logged in in a else no directly name

Question

this is the whole code in stoerer.php

<!doctype html>
<html lang="de">
<meta charset="utf-8">
<title> Störer</title>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
<link rel="stylesheet" href="style.css" type="text/css">

</head> 

<body>
    <header> 
<button>Ich bin ein Störer</button>



<!-- Und dann die Info-Box --> 
<div id="infoBox">
<button class="cross" type="button">X</button>
  <p> Hello </p> <?php wp_get_current_user(); ?>




  <p>  Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gub

  </p>
</div>
</header> 

<script>
$("button").click(function(){
    $("#infoBox").toggle();
  });

</script>
</body>
</html>

this is the whole code in the child theme style.css:

/*
 Theme Name:   Twenty Twenty Child
 Description:  Mein Child Theme
 Author:       Webtimiser
 Author URI:   https://www.example.com
 Template:     twentytwenty
 Version:      1.0
 Text Domain:  twenty-twenty-child
*/

and this is the functions.php in the child theme:

<?php
/**
* Child theme stylesheet einbinden in Abhängigkeit vom Original-Stylesheet
*/

function child_theme_styles() {
wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
wp_enqueue_style( 'child-theme-css', get_stylesheet_directory_uri() .'/style.css' , array('parent-style'));

}
add_action( 'wp_enqueue_scripts', 'child_theme_styles' );



add_shortcode( 'get_current_user' , 'wp_get_current_user_func' );

function wp_get_current_user_func( $atts ) {

    $current_user = wp_get_current_user();

    if ($current_user != 0)
    {
        return $current_user->user_firstname;
    } else
    {
        return ""
    }
}?>

and it’s still the same error