I've try to replace incoming POST by a variable but, its does not working

$formErrors = array();
$ve = $_POST['username'];  //(line 14) i want to replace this by variable but not work
// Validation of Username
if (isset($_POST['username'])) {
  $filterUser = filter_var($_POST['username'], FILTER_SANITIZE_STRING);
}

Warning: Undefined array key "username" in C:\xampp\htdocs\eCommerce\signup.php on line 14

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