How do I fix this problem

[Husam]

Hi everybody:

when I make the web.config file to make the authentication and the
autherization for my web site.

the code is as following:

<configuration>
<appSettings/>
<connectionStrings/>
<system.web>
<authentication mode="Forms">
<forms name="Default" loginUrl="~/Default.aspx" />
</authentication>
</system.web>
</configuration>

But when I dubbge my page I recive the following message:

It is an error to use a section registered as
allowDefinition='MachineToApplication' beyond application level. This error
can be caused by a virtual directory not being configured as an application
in IIS.

the red line is :

<authentication mode="Forms">