while adding web reference

Question

       I get an error when i try to add a web reference in a web site using C#.NET
      The error message that i get is

Server Error in '/' Application.

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Configuration Error

Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

Parser Error 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.

Source Error:

Line 24:             ASP.NET to identify an incoming user. 
Line 25:         -->
Line 26: 		<authentication mode="Forms"/>
Line 27: 		<!--
Line 28:             The <customErrors> section enables configuration 

Source File: c:\inetpub\wwwroot\author\web.config    Line: 26

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Version Information: Microsoft .NET Framework Version:2.0.50727.42; ASP.NET Version:2.0.50727.42