Get parent menu item of a current node
I think you need to reset the collection first, as the ::loadLinksByRoute() annotation states that it returns an array of MenuLinkInterfaces[].
See /core/lib/Drupal/Core/Menu/MenuLinkManagerInterface.php:

  /**
   * Loads multiple plugin instances based on route.
   *
   * @param string $route_name
   *   The route name.
   * @param array $route_parameters
   *   (optional) The route parameters. Defaults to an empty array.
   * @param string $menu_name
   *   (optional) Restricts the found links to just those in the named menu.
   *
   * @return \Drupal\Core\Menu\MenuLinkInterface[]
   *   An array of instances keyed by plugin ID.
   */
  public function loadLinksByRoute($route_name, array $route_parameters = [], $menu_name = NULL);

So do something like:
/** @var \Drupal\Core\Menu\MenuLinkManagerInterface $menu_link_manager */
$menu_link_manager = \Drupal::service('plugin.manager.menu.link');
$menu_links = $menu_link_manager-&gt;loadLinksByRoute('entity.node.canonical', array('node' =&gt; $node_id));
// Reset collection by retrieving the first item.
$menu_link = reset($menu_links);
// Now call the parent method.
$parent = $menu_link-&gt;getParent();