Sub menu does not work on WP how to solve the problem?

Question

WordPress

Sub menu does not work on WP if you drag and drop through the admin panel. How can I make a sub menu, hover or click option? And somehow highlight that there is also a sub menu on the link, maybe some kind of before arrow? Menu WP, there is jQuery.

Code part:

<nav aria-label="Horizontal" role="navigation">
<ul class="menu">
 <li id="menu-item-56" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-has-children menu-item-56 nav-item"><a href="/" class="nav-link">Services</a><span class="icon"></span>
<ul class="sub-menu">
<li id="menu-item-299" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-299 nav-item"><a href="/" class="nav-link">Discovery phase</a></li>
</ul>
</li>

It is necessary by clicking or hover (whereas on the tablet and mobile version indicate that there is a sub menu) on li id="menu-item-56" sub menu dropped down, li id="menu-item-299" with a white background.