There is a well documented error for XML Sitemap generated by Yoast SEO Plugin which leads to the XML Sitemap not found and a 404 Page not found error generated. The details of the error and its possible resolution can be found at the following URL
Solution for error for XML Sitemap
This site also faced similar problem while generating sitemap for submission with Google Search Console. So the immediate reaction was to use another plugin which successfully generated the sitemap. After a detailed study and a bit of troubleshooting, what team SBW found that if your site has no content at all then sitemap will not be generated by this plugin. So once a post was created and published, the plugin started working fine.
Hope this tip will help users who have not been able to generate sitemaps using Yoast SEO.