OK, I see. There’s no excerpt option for the page by default in WordPress, if you need to use this option, you can add it by the following custom code.
add_action( 'init', 'miao_add_excerpts_to_pages' );
function miao_add_excerpts_to_pages() {
add_post_type_support( 'page', 'excerpt' );
}
By the way, you can install this plugin https://wordpress.org/plugins/custom-css-js-php/ then add those php code through this plugin. Then, the codes will not lost when you update the theme in the future.
Thanks