Your Yoast SEO-generated sitemap at /sitemap_index.xml/ is an XML file, as expected. Please ensure that you’ve submitted only that.
I’m unsure why Google reports that your sitemap is in HTML format. However, I suspect that this could be due to the cache. Try clearing your cache from your plugin, theme, server, browser, or CDN, such as Cloudflare and?remove and resubmit the sitemap. If you are not sure how to clear your server caching, please speak with your host provider. If you need to clear your browser caching, this guide may help:?How to clear my browser’s cache?
Please also remember that for any sitemap changes, it might take a while for Google to reindex your site and provide an update report in Google Search Console (as these reports?are not real-time).