web.config authentication problem

[Mr. x]

I have in the web.config file something like this :

<configuration>
<system.web>
<authentication mode="Forms">
<forms name=".ASPXAUTH" loginUrl="main.aspx" protection="All"
timeout="1" path="/" requireSSL="false" slidingExpiration="true">
</forms>
<passport redirectUrl="internal" />
</authentication>
</system.web>
</configuration>

When I try to open main.aspx I get the error :
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.

The same error is on my computer, and also on hosting computer.

Why ?

Thanks