IndexNow is a paid feature

Hi!

While IndexNow is indeed open source and free to use, we still have to develop and tap into the API to ensure it works with the Yoast SEO plugin. You can learn more about it here – https://yoast.com/what-is-indexnow/

But even with the free version of Yoast SEO, you can be assured that whenever you add, remove, or modify content, your sitemap (and the last modified date!) is updated automatically and will let search engines know about changes on the content on your site accordingly.