Hi Lex,
Thanks for getting back to us.
We checked your sitemap https://hotelornex.fr/sitemap_index.xml and it is loading as expected. So we’re not sure why Google is reporting an issue with the format.
Can you confirm you’re submitting “sitemap_index.xml” as we describe in this article https://kb.yoast.com/kb/submit-sitemap-search-engines/#google?
Before doing so, we recommend deleting the current sitemap and clearing the cache from your theme, plugin, server, CDN like CloudFlare and browser. If you are not sure how to clear caching from theme/plugin please speak to those authors. If you want to clear cache from the server, please speak to your host provider. To clear the cache from a browser use this guide: https://kb.yoast.com/kb/how-to-clear-my-browsers-cache/.
We look forward to your response.