sitemap 404 not found

Hi guys, great plugin

I have ticked the Check this box to enable XML sitemap functionality box but the url it gives me

https://bitcoin-betting-guide.com/sitemap_index.xml

Is a 404 not found, google webmaster tools confirms there not sitemap at that address.

How to fix?

Thanks

https://www.ads-software.com/plugins/wordpress-seo/