While upgrading the StoreFront server from 1903 to 1912.CU1, we get below error
" An error occurred during installation . Please ensure all the required prerequisites have been installed - Citrix storefront"

In the StoreFront we disabled XenApp Services Support. Right-click the store, and click Configure XenApp Services Support and uncheck: Enable XenApp Services Support.

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Problem Cause

MSI logs shows:

Begin SnapshotConfiguration
Creating version snapshot for version '3.18.0.21021'.
Loading backup history configuration
Loading backup configuration from: 'C:\ProgramData\Citrix\Storefront Install\3.18.0.21021'
Incomplete snapshot exists for version '3.18.0.21021' so deleting.
No snapshot exists for version '3.18.0.21021', create one now.
An error occurred creating the snapshot: 'System.IO.FileNotFoundException: ConfigFileNotFound
File name: 'C:\inetpub\wwwroot\Citrix\PNAgent\web.config'
   at Citrix.DeliveryServices.InstallController.Configuration.ConfigFileValidator.Load()
   at Citrix.DeliveryServices.InstallController.Configuration.ConfigFileValidator..ctor(String configFilePath)
   at Citrix.DeliveryServices.InstallController.Configuration.VersionData.BackupConfigFile(IFeatureInstance instance, DirectoryInfo instanceBackup)
   at Citrix.DeliveryServices.InstallController.Configuration.VersionData.BackupFeatureInstances()
   at Citrix.DeliveryServices.InstallController.Configuration.VersionData.Creat