Filter WordPress category archive to display Learndash Lessons

functions.php

<?php
function filter_category_archive_to_display_lessons( $query ) {
    if ( ! is_admin() && $query->is_main_query() ) {
        // Not a query for an admin page.
        // It's the main query for a front end page of your site.
 
        if ( is_category() ) {
            // It's the main query for a category archive.
 
            // Let's change the query for category archives to only display lessons.
            $query->set( 'post_type', 'sfwd-lessons' );
        }
    }
}
add_action( 'pre_get_posts', 'filter_category_archive_to_display_lessons' );