How to fix authentication error in a sub folder?

How to fix authentication error in a sub folder?

Server Error in '/' Application.
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 8:         <compilation debug="true">
Line 9:         </compilation>
Line 10:        <authentication mode="Windows"/>
Line 11:        <customErrors mode="Off"/>
Line 12:    </system.web>


Source File: d:hostingmydomainmysubfolderweb.config    Line: 10

Version Information: 

Microsoft .NET Framework Version:2.0.50727.3625; ASP.NET Version:2.0.50727.3634


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