Lo tenes comentado esa parte... copia este.
Elimina los "<!--" y "-->" en esa parte para que quede solo esto
Borra el texto
<!--
The <authentication> section enables configuration
of the security authentication mode used by
ASP.NET to identify an incoming user.
-->
<authentication mode="Windows"/>
<!--
The <customErrors> section enables configuration
of what to do if/when an unhandled error occurs
during the execution of a request. Specifically,
it enables developers to configure html error pages
to be displayed in place of a error stack trace.
<customErrors mode="Off">
</customErrors>
-->
Que quede solo esto
<authentication mode="Windows"/>
<customErrors mode="Off"></customErrors>