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.
Simple Way to fix it in ASP.Net
In Web config, add below code, out of system.web
<system.webServer>
<modules runAllManagedModulesForAllRequests="true">
</modules>
</system.webServer>