Your endpoint is wrong, /wp/v2/posts is just for objects with post_type="post". If your custom post type is registered with ‘activity-api’ the endpoint would be /wp/v2/activity-api/11.
That’s because it’s the slug you registered in 'rest_base' => 'activity-api' argument to your post type. Without a custom rest_base argument, the endpoint would be at /wp/v2/st_activity