Related Posts

Hello GP,

I'm trying to add related post after a blog post without using a plugin.
I follow what I see on this site: https://www.wplogout.com/display-related-posts-in-generatepress-theme/

I make a hook and add this code

<div class="wpsp-related-posts1  grid-container">
	<h2>Related Posts</h2>
<?php
if ( is_single() ) {
    $cats =  get_the_category();
    $cat = $cats[0];
} else {
    $cat = get_category( get_query_var( 'cat' ) );
}

$cat_slug = $cat->slug;
$list = get_page_by_title( 'related', 'OBJECT', 'wp_show_posts' );
wpsp_display( $list->ID, 'tax_term="' . $cat_slug . '"' );
?>
</div>

It says nothing can be found.

I proceed to add this code to show related post determined by tags and get a weird error on line 7.

<div class="wpsp-related-posts1  grid-container">
	<h2>Related Posts</h2>
<?php    
if ( is_single() ) {
        $tags =  get_the_tags();
        $tags_list = [];
        foreach ($tags as $tag)
            $tags_list[] = $tag->slug;
        $tag_string = implode( ', ', $tags_list);
    } else {
        $tag_string = get_tag( get_query_var( 'tag' ) );
    }

    $list = get_page_by_title( 'related', 'OBJECT', 'wp_show_posts' );
    wpsp_display( $list->ID, 'tax_term="' . $tag_string . '"' );
    ?>
</div>

Can anyone tell me what I'm doing wrong?

Is there any alternative to add related posts?