Call to a member function execute() on boolean

<?php

 // à mettre tout en haut du fichier .php, cette fonction propre à PHP servira à maintenir la $_SESSION
session_start();
include("config.php");

$error = false;
$adminpageError = "";
$code4Error = "";
$code4noncorrespondError = "";

if (isset($_SESSION['username']) && $_SESSION['username'] =="admin") {
  $adminpageError = "<p><a href='inscription.php'>Admin page </a></p>";
}

if(isset($_POST['valider'])){
  if (empty($_POST['code'])) {
    $error = true;
    $codetransfert4Error = "Please enter the transfer code .";
  } else {
    $code4 = htmlentities($_POST['code4'], ENT_QUOTES, "ISO-8859-1");
    $mysqli = mysqli_connect("localhost", "id4295068_roomadmin", "", "id4295068_projet");
    if (!$mysqli) {
      echo "Please check your information and try again";
    } else {
      $query = "SELECT * FROM `client6` WHERE code4='".$code4."'";
      $result = mysqli_query($mysqli,$query) or die(mysqli_error());
      $rows = mysqli_num_rows($result);
      if($rows==1) {
        header("location: load4.php");
      } else {
        $query = "SELECT * FROM `client6` WHERE  code4='".$code4."'";
        $result = mysqli_query($mysqli,$query) or die(mysqli_error());
        $rows = mysqli_num_rows($result);
        if($rows==0) {
          header("location: transfert4non.php");
        }
      }
    }
  }
}
// Si tout va bien, on peut continuer
// On récupère tout le contenu de la table client6
$req = $mysqli->prepare('SELECT * FROM `client6` WHERE username = :username');
$req->execute(array('username' => $_SESSION['username']));

// On affiche chaque entrée une à une
while ($donnees = $req->fetch())

?>