custom post type taxonomy “tag” archive : no post found

Tag and Category archive queries default to querying only the post post type, to add your custom post type to those queries, you can use the pre_get_posts action:

function wpa_cpt_tags( $query ) {
    if ( $query->is_tag() && $query->is_main_query() ) {
        $query->set( 'post_type', array( 'post', 'object' ) );
    }
}
add_action( 'pre_get_posts', 'wpa_cpt_tags' );