你的 CSS 错误。
li
没有课wp-submenu
ul.wp-submenu li:nth-child(4) {
display: none;
}
<ul class="wp-submenu wp-submenu-wrap">
<li class="wp-submenu-head" aria-hidden="true">Media</li>
<li class="wp-first-item"><a href="upload.php" class="wp-first-item">Library</a></li>
<li><a href="media-new.php">Add New</a></li>
<li><a href="upload.php?page=webpc_optimization_page">Converter for Media</a></li>
</ul>