added required PowerShell version

Author: tfitzmac

articles/azure-resource-manager/templates/template-deploy-what-if.md

@@ -3,7 +3,7 @@ title: Template deployment what-if (Preview)
 description: Determine what changes will happen to your resources before deploying an Azure Resource Manager template.
 author: mumian
 ms.topic: conceptual
-ms.date: 04/09/2020
+ms.date: 04/27/2020
 ms.author: jgao
 ---
 # ARM template deployment what-if operation (Preview)
@@ -17,7 +17,9 @@ You can use the what-if operation with the PowerShell commands or REST API opera
 
 ## Install PowerShell module
 
-To use what-if in PowerShell, install a preview version of the Az.Resources module from the PowerShell gallery.
+To use what-if in PowerShell, you must have PowerShell Core (6.x or 7.x). If you have PowerShell 5.x or earlier, [update your version of PowerShell](/powershell/scripting/install/installing-powershell).
+
+After making sure you have the correct version of PowerShell, install a preview version of the Az.Resources module from the PowerShell gallery.
 
 ### Install preview version