Citrus, welcome to TalkGraphics.

Your <url>, <lastmod> and file format were wrong.

Try:
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url>
<loc>https://byflury.ch.html</loc>
<lastmod>2024-04-09</lastmod>
</url>
</urlset>

Xara produces your sitemap/xml (& robots.txt) file for you: https://helpandsupport.xara.com/xara...tive_host.html
Put https://byflury.ch.html into the Website URL box. No need to use the Google Search Console at all.

Acorn