How to assign post to category

Question

I have created custom post type and created two post category This Month and Next Month and assigned sample posts to each category. When I preview the category archive ../category/this-month a page said Nothing Found so I assume that the page exist but the post is not display. The question is how can I make any post that is assign to post category show on say /category/this-month?

// Register Custom Post Type
function activity() {

    $labels = array(
        'name'                  => _x( 'Activity', 'Post Type General Name', 'twenty_two' ),
        'singular_name'         => _x( 'Activity', 'Post Type Singular Name', 'twenty_two' ),
        'menu_name'             => __( 'Activity', 'twenty_two' ),
        'name_admin_bar'        => __( 'Activity', 'twenty_two' ),
        'archives'              => __( 'Item Archives', 'twenty_two' ),
        'attributes'            => __( 'Item Attributes', 'twenty_two' ),
        'parent_item_colon'     => __( 'Parent Product:', 'twenty_two' ),
        'all_items'             => __( 'All Activity', 'twenty_two' ),
        'add_new_item'          => __( 'Add New Activity', 'twenty_two' ),
        'add_new'               => __( 'New Activity', 'twenty_two' ),
        'new_item'              => __( 'New Item', 'twenty_two' ),
        'edit_item'             => __( 'Edit Activity', 'twenty_two' ),
        'update_item'           => __( 'Update Activity', 'twenty_two' ),
        'view_item'             => __( 'View Activity', 'twenty_two' ),
        'view_items'            => __( 'View Items', 'twenty_two' ),
        'search_items'          => __( 'Search Activity', 'twenty_two' ),
        'not_found'             => __( 'No Activity found', 'twenty_two' ),
        'not_found_in_trash'    => __( 'No products found in Trash', 'twenty_two' ),
        'featured_image'        => __( 'Featured Image', 'twenty_two' ),
        'set_featured_image'    => __( 'Set featured image', 'twenty_two' ),
        'remove_featured_image' => __( 'Remove featured image', 'twenty_two' ),
        'use_featured_image'    => __( 'Use as featured image', 'twenty_two' ),
        'insert_into_item'      => __( 'Insert into item', 'twenty_two' ),
        'uploaded_to_this_item' => __( 'Uploaded to this item', 'twenty_two' ),
        'items_list'            => __( 'Items list', 'twenty_two' ),
        'items_list_navigation' => __( 'Items list navigation', 'twenty_two' ),
        'filter_items_list'     => __( 'Filter items list', 'twenty_two' ),
    );
    $args = array(
        'label'                 => __( 'Activity', 'twenty_two' ),
        'description'           => __( 'Display activities post', 'twenty_two' ),
        'labels'                => $labels,
        'supports'              => array( 'title', 'editor', 'thumbnail' ),
        'taxonomies'            => array( 'category', 'post_tag' ),
        'hierarchical'          => false,
        'public'                => true,
        'show_ui'               => true,
        'show_in_menu'          => true,
        'menu_position'         => 5,
        'show_in_admin_bar'     => true,
        'show_in_nav_menus'     => true,
        'can_export'            => true,
        'has_archive'           => 'activities',
        'exclude_from_search'   => false,
        'publicly_queryable'    => true,
        'capability_type'       => 'page',
    );
    register_post_type( 'activity', $args );

}
add_action( 'init', 'activity', 0 );

Answer 1

You have to register built-in "category" taxonomy for the "Activity" post type:

<?php
function add_categories_to_activities()
{
    register_taxonomy_for_object_type( 'category', 'activity' );
}

add_action( 'init', 'add_categories_to_activities' );

See register_taxonomy_for_object_type() in the Code Reference.