Session state can only be used when enableSessionState is set to true, either in a configuration file or in the Page directive- SharePoint2010

Complete Error Message

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.

Before you start your troubleshooting, make sure that the State Service service application is running. How you will check this?

• Open your Central Administration
• Manage service applications
• Check the service application by the name ‘state service service application’

Resolution

Make the enableSessionState is set to true by following below step

<pages enableSessionState="true" enableViewState="true" enableViewStateMac="true" validateRequest="false" pageParserFilterType="Microsoft.SharePoint.ApplicationRuntime.SPPageParserFilter, Microsoft.SharePoint, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" asyncTimeout="7">
Make sure that the remove and add lines exist in the modules section for Session.

<modules runAllManagedModulesForAllRequests="true">

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

After implementing the above changes, the error should have to go away.

Product Applies To

SharePoint Server 2010

SharePoint Foundation 2010

References
What is session state? What exactly its functionality? How it works?

http://msdn.microsoft.com/en-us/library/ms178581%28v=vs.100%29.aspx

This report failed to load because session state is not turned on" when you try to view Microsoft Access Services reports

http://support.microsoft.com/kb/2419469
State service and session state cmdlets (SharePoint Server 2010)

http://technet.microsoft.com/en-in/library/ee890113(v=office.14).aspx
Manage the State Service (SharePoint Server 2010)

http://technet.microsoft.com/en-us/library/ee704548(v=office.14).aspx