How to use If, else condition in wordpress

I am building a new child theme and I came across a problem which I think you all can help me with. I am new to WordPress and is stuck at this point. I am using theme simone { https://wordpress.org/themes/simone } for this purpose.

the problem is that I want to show sub-categories of parents till there is no sub-categories left and then on the last sub-category it should show the posts it contains. For that I have prepared two codes separately but when I combine them they give me error. Please help me with this. as separate code they work as I want. But when combined they give error.

Code : 1 { for showing sub-categories.

<ul>
<?php
	$cat = get_query_var('cat');
	$category = get_category ($cat);

	if ($category->cat_ID) {
		$children = wp_list_categories("orderby=id&depth=0&hide_empty=0&title_li=&child_of=".$category->cat_ID."&echo=0");
		// change depth to 1 if only the top level sub categories should be shown i.e., with children NOT expanded
		if ($children) {
			echo $children;
		}

	}
?>
</ul>

Code : 2 { for showing the posts under the sub-category }

<ul>
<?php
global $post;
$category = get_the_category($post->ID);
$category = $category[0]->cat_ID;
$myposts = get_posts(array('numberposts' => 20, 'offset' => 0, 'category__in' => array($category), 'post_status'=>'publish', 'order'=>'ASC' ));
foreach($myposts as $post) :
setup_postdata($post);
?>
<li>
<a href="<?php the_permalink(); ?>">
<?php the_title(); ?></a>
</li>
<?php endforeach; ?>
</ul>

please help me combining these two codes so that they work properly.