SessionState

Session state can only be used when enableSessionState is set to true, either in a configuration file or in the Page directive. Please also make sure that System.Web.SessionStateModule or a custom session state module is included in the <configuration>\<system.web>\<httpModules> section in the application configuration.

Change the Web.Config as follow:

Add the following under modules:

<add name="Session" type="System.Web.SessionState.SessionStateModule" preCondition="" />

modules:

<modules runAllManagedModulesForAllRequests="true">
...
</modules>

Note: 'web.config' can normaly be found under

C:\inetpub\wwwroot\wss\VirtualDirectories\<portnumber>\web.config