How to fix Google Webmaster XML Sitemap error

Question

I am getting error on Google search console for XML Sitemap.

Sitemap.XML

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://sitemaps.org/schemas/sitemap/0.9">
<url>
    <loc>https://example.com/channels/</loc>
    <changefreq>daily</changefreq>
    <priority>0.8</priority>
</url>
</urlset>

Error: Your Sitemap or Sitemap index file doesn't properly declare the namespace.

Sitemap Index XML:

<?xml version="1.0" encoding="UTF-8"?>
<sitemapindex xmlns="http://sitemaps.org/schemas/sitemap/0.9">
<sitemap>
<loc>https://example.com/sitemap_main.xml</loc>
</sitemap>
</sitemapindex>

Error : Your Sitemap or Sitemap index file doesn't properly declare the namespace.