The following error is displayed when upgrading StoreFront 3.0 to 3.6:
An error occurred during installation. Please ensure all the required prerequisites have been installed and run the installer again.

1. Checked file CitrixMsi-CitrixStoreFront-x64 and found below exception:

   Begin SnapshotConfiguration Creating version snapshot for version '3.0.1000.1'. Loading backup history configuration Loading backup configuration from: 'C:\ProgramData\Citrix\Storefront Install\2.6.0.5031' Loading backup configuration from: 'C:\ProgramData\Citrix\Storefront Install\3.0.1000.1' Snapshot exists for version '3.0.1000.1' so deleting. An error occurred creating the snapshot: 'System.Configuration.ConfigurationErrorsException: An error occurred creating the configuration section handler for citrix.deliveryservices/protocolTransitionSettings: Could not load file or assembly 'Citrix.DeliveryServices.ProtocolTransition.Configuration, Version=2.6.0.0, Culture=neutral, PublicKeyToken=e8b77d454fa2a856' or one of its dependencies. line The system cannot find the file specified. (C:\Program Files\Citrix\Receiver StoreFront\Services\ProtocolTransitionService\Citrix.DeliveryServices.ProtocolTransition.ServiceHost.exe.config9) ---> System.IO.FileNotFoundException: Could not load file or assembly 'Citrix.DeliveryServices.ProtocolTransition.Configuration, Version=2.6.0.0, Culture=neutral, PublicKeyToken=e8b77d454fa2a856' or one of its dependencies. The system cannot find the file specified. at System.Configuration.TypeUtil.GetTypeWithReflectionPermission(IInternalConfigHost host, String typeString, Boolean throwOnError) at System.Configuration.MgmtConfigurationRecord.CreateSectionFactory(FactoryRecord factoryRecord) at System.Configuration.BaseConfigurationRecord.FindAndEnsureFactoryRecord(String configKey, Boolean& isRootDeclaredHere)

2. We navigated to C:\Program Files\Citrix\Receiver StoreFront\Services\ProtocolTransitionService and created a backup copy of Citrix.DeliveryServices.ProtocolTransition.ServiceHost.exe.config file.

3. We replaced all 2.6.0.0 with 3.0.0.0 and saved the file.

4. Ran the installation again and it went fine.