Category Archive not working for pages

Your code did work for me; I was able to see pages under the category archive for Association Landing Pages at http://domain.com/category/association-landing-pages.

It sounds like you need to use the URL:

localhost/mywordpressitefolder/category/association-landing-pages

I noticed that you said that the slug was association-landing-pages and then you visited localhost/mywordpressitefolder/association-landing-page (without the s). Make sure to use the right URL based on your slug, and add /category too, as explained above.

I would also suggest putting your is_admin() check inside the category_and_tag_archives() function. I made a couple of other tweaks to the original code too:

function add_taxonomies_to_pages() {
    register_taxonomy_for_object_type( 'category', 'page' );
}
add_action( 'init', 'add_taxonomies_to_pages' );

function category_and_tag_archives( $wp_query ) {
    $my_post_array = array( 'post', 'page' );

    if ( ! is_admin() && is_category() && $wp_query->is_main_query() ) {
        $wp_query->set( 'post_type', $my_post_array );
    }
}
add_action( 'pre_get_posts', 'category_and_tag_archives' );