The most common case when the “The file web.sitemap required by XmlSiteMapProvider does not exist” error is received is when a SiteMap control was added to a WebForm, however you don’t have a mandatory Web.sitemap file in the root of your web application.
To fix this error, make sure you have this file in the root directory, and that it is a valid XML file that has the website’s sitemap defined. An example Web.sitemap file can be seen below:

<?xml version=“1.0” encoding=“utf-8” ?>

<siteMap xmlns=“http://schemas.microsoft.com/AspNet/SiteMap-File-1.0” >

    <siteMapNode url=“Default.aspx” title=”Geekpedia”  description=“Home Page of Geekpedia.com “>

        <siteMapNode url=“Register.aspx” title=“Registration”  description=“Register a new account.” />

        <siteMapNode url=”SignIn.aspx” title=”Sign-In”  description=“Sign-in into your existing account.“ />

    </siteMapNode>

</siteMap>