Adding Custom Taxonomy to WordPress default Post type

I am trying to add custom taxonomy (tags) to WordPress default post type. because I wanna use that taxonomy for custom post type as well.

// Register Custom Taxonomy
function tag_synchro() {

$labels = array(
    'name'                       => _x( 'QA Tags', 'Taxonomy General Name', ET_DOMAIN ),
    'singular_name'              => _x( 'QA Tag', 'Taxonomy Singular Name', ET_DOMAIN ),
    'menu_name'                  => __( 'QA Tags', ET_DOMAIN ),
    'all_items'                  => __( 'All Tags', ET_DOMAIN ),
    'parent_item'                => __( 'Parent Item', ET_DOMAIN ),
    'parent_item_colon'          => __( 'Parent Item:', ET_DOMAIN ),
    'new_item_name'              => __( 'New Tags Name', ET_DOMAIN ),
    'add_new_item'               => __( 'Add New tag', ET_DOMAIN ),
    'edit_item'                  => __( 'Edit tag', ET_DOMAIN ),
    'update_item'                => __( 'Update tag', ET_DOMAIN ),
    'separate_items_with_commas' => __( 'Separate tags with commas', ET_DOMAIN ),
    'search_items'               => __( 'Search Items', ET_DOMAIN ),
    'add_or_remove_items'        => __( 'Add or remove items', ET_DOMAIN ),
    'choose_from_most_used'      => __( 'Choose from the most used tagd', ET_DOMAIN ),
    'not_found'                  => __( 'Not Found', ET_DOMAIN ),
);
$args = array(
    'labels'                     => $labels,
    'hierarchical'               => false,
    'public'                     => true,
    'show_ui'                    => true,
    'show_admin_column'          => true,
    'show_in_nav_menus'          => true,
    'show_tagcloud'              => true,
    'update_count_callback'      => '_update_post_term_count',
    'query_var'             => true,
    'rewrite'               => array( 'slug' => ae_get_option('tag_slug', 'qa-tag') ),
    );
register_taxonomy( 'qa_tag', array( 'post' ), $args );

}

// Hook into the 'init' action
add_action( 'init', 'tag_synchro', 0 );

I used above code, and it’s seems not working. is there any way I can accomplish this work. Thank you in advance

Try this

add_action( 'init', 'tag_synchro', 10 );

It might be the case that, when you set priority to 0 , your callback does not get called.