Related Content – as a WIDGET

Hey Julie,
Thank you for your patience, unfortunately, you can not do this typically, but you could try this with a custom shortcode and some custom CSS. First, try adding this code to the end of your functions.php file in Appearance > Editor:

function related_posts_shortcode( $attr ) {
    ob_start();
    get_template_part( 'includes/related-posts' );
    return ob_get_clean();
}
add_shortcode( 'rpost', 'related_posts_shortcode' );

I recommend using a child theme

Then add a Custom HTML widget to your sidebar and use this shortcode: [rpost]

Then add this css to your child theme stylesheet, or theme Quick CSS area:

.widget_custom_html > .custom-html-widget > .related_posts {
	padding: 23px 0 0 0;
}
#top .widget_custom_html > .custom-html-widget > .related_posts > .related_entries_container > .related_column {
    width: 100%;
}
#top main > .related_posts {
	display: none;
}

Please note that the last rule is to hide the regular related posts element from your posts.
Expected results:

Best regards,
Mike