Your Sitemap or Sitemap index file does not properly declare the namespace!

Question

This is my sitemap, it starts like this:

<?xml version="1.0" encoding="UTF-8"?>

<urlset
      xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
      xmlns:xhtml="http://www.w3.org/1999/xhtml/"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9
            http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd">


<!-- Domain Name Policy (DNP) in: https://github.com/jfoclpf/autocosts/wiki/Domain-name-policy -->

    <url>
        <loc>https://autocostos.info/AR</loc>
        <changefreq>weekly</changefreq>
        <xhtml:link rel="alternate" hreflang="x-default"
                    href="https://autocosts.info/" />

            <xhtml:link rel="alternate" hreflang="es-AR"
                        href="https://autocostos.info/AR" />
            <xhtml:link rel="alternate" hreflang="en-AU"
                        href="https://autocosts.info/AU" />
            <xhtml:link rel="alternate" hreflang="es-BO"
                        href="https://autocostos.info/BO" />
            <xhtml:link rel="alternate" hreflang="pt-BR"
                        href="https://autocustos.info/BR" />
            <xhtml:link rel="alternate" hreflang="en-CA"
                        href="https://autocosts.info/CA" />
            <xhtml:link rel="alternate" hreflang="es-CL"
                        href="https://autocostos.info/CL" />
            <xhtml:link rel="alternate" hreflang="zh-CN"
                        href="https://autocosts.info/CN" />
            <xhtml:link rel="alternate" hreflang="es-CO"
                        href="https://autocostos.info/CO" />
            <xhtml:link rel="alternate" hreflang="es-CR"
                        href="https://autocostos.info/CR" />

But Google Search console insists that

Your Sitemap or Sitemap index file does not properly declare the namespace

They just give an example on line 16: Line 16 Parent tag: url Tag: link

Line 16 corresponds to <xhtml:link rel="alternate" hreflang="x-default" href="https://autocosts.info/" />

What am I doing wrong?

Answer 1

After searching a lot, I found the solution, I had to have the following urlset:

<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:xhtml="http://www.w3.org/1999/xhtml"
    xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9
    http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd
    http://www.w3.org/1999/xhtml
    http://www.w3.org/2002/08/xhtml/xhtml1-strict.xsd">