@stavroch,
We are not sure why Google says that. We see that your sitemap here: https://www.suco.gr/sitemap_index.xml does show urls on it. We ask the following three actions:
1.Does clearing your caching from your plugin, theme, server, browser, or CDN such as Cloudflare resolve the issue? If you are not sure how to clear your server caching, please speak with your host provider.
2.Try to resubmit the sitemap. This guide explains more: https://kb.yoast.com/kb/submit-sitemap-search-engines/#google
3. If errors still appear, we suggest contacting Google as the sitemap does contain urls. You may?contact Google by making a post on their forums: https://productforums.google.com/forum/?rd=1#!forum/webmasters.?
@sarithareddy,
We are not sure why Google says that. We can see your sitemap appears here: https://www.eknowledgetree.com/sitemap_index.xml with urls on it. We suggest the actions described above as your sitemap has urls on it and is in the correct format.