The xsd that should validate the sitemap.xml generated by Liferay can be found at https://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd. This xsd file specifies that, to pass validation, any tags using a namespace different than "http://www.sitemaps.org/schemas/sitemap/0.9" should specify the schema location for that namespace. It is enforce by the value "strict" below.
If the schema location is not set withing the tag <urlset> of sitemap.xml the following error occurs at validation (use for instance the following online validation tool: https://www.corefiling.com/opensource/schemavalidate/)
To reproduce it, use the xsd file from the first url above and a sitemap obtained for a liferay site and pass it through the validation tool.
Comment: To point to the schema location used by xmlns:xhtml="http://www.w3.org/1999/xhtml" the complete tag <urlset> should be: