Register non hierarchical taxonomy and the show meta box in the post type

The meta_box_cb argument is expected to be a callback function. So it should be a callable.

As Milo suggested in his comment, try to leave that argument away, then the default function for the metabox content will be used. You only need to add an own metabox function if you actually want to customize the metabox output.

    $args = array(
        'hierarchical'          => false,
        'labels'                => $labels,
        'show_ui'               => true,
        'show_admin_column'     => true,
        'update_count_callback' => '_update_post_term_count',
        'query_var'             => true,
        'rewrite'               => array( 'slug' => 'tags' ),
    );

As described in the Code Reference, post_tags_meta_box is the default value for non-hierarchical taxonomies. The default value is used, if the argument is left away. As it is explicitely mentioned as being optional, it is perfectly ok to just leave it away.