Authentication only for one page

Hi everyone,
The error:
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.



what I'm doing:

I need to enable forms authentication for only one of my pages.
so I made an application under my project using IIS and enabled its forms Authentication. The application contains the page that requires user to log in and a web.config. Below is the code of web.config under the application. 
<?xml version="1.0"?> <configuration> <appSettings/> <connectionStrings/> <system.web> <authentication mode="Forms"> <forms loginUrl="login.aspx" protection="All" t ...