Get post/page data outside the loop

You’re using non-existent variables in your function, $post_id, $post, $page.

Use get_queried_object to get data from the current page.

if( is_single() || is_page() ){
    $this_page = get_queried_object();
    $excerpt = get_post_field( 'post_excerpt', $this_page->ID );
}

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