How to add tag in pages?

Hi there,

the Theme has no control over that, adding taxonomies is a core WordPress function.
I am not sure why that plugin isn't working for you.

You can try using this PHP Snippet instead of the plugin:

function add_tags_to_pages() {
    register_taxonomy_for_object_type( 'post_tag', 'page' );
}
add_action( 'init', 'add_tags_to_pages');

How to add PHP: https://docs.generatepress.com/article/adding-php/

Thanks

You're welcome