Azure自定义扩展失败,错误消息为:“已完成执行命令”

我开始在Azure中测试自定义扩展,并从非常简单的json和ps1脚本开始。我的json文件如下所示:

{
"$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
"contentVersion": "1.0.0.0",
"parameters": {
    "vmName": {
        "type": "string"
    }
},
"resources": [
    {
        "type": "Microsoft.Compute/virtualMachines/extensions",
        "name": "[concat(parameters('vmName'),'/RunDateTimeScript')]",
        "apiVersion": "2015-06-15",
        "location": "[resourceGroup().location]",
        "properties": {
            "publisher": "Microsoft.Compute",
            "type": "CustomScriptExtension",
            "typeHandlerVersion": "1.8",
            "autoUpgradeMinorVersion": true,
            "settings": {
                "fileUris": [
                    "https://github.com/username/repo/blob/master/writedatetime.ps1"
                ],
                "commandToExecute": "powershell.exe -ExecutionPolicy Unrestricted -File writedatetime.ps1"
            }
        }
    }
]
}

我的powershell脚本是一个单行程序:

(Get-Date).DateTime | Out-File "C:\Testing.txt" -Append

但是,当我运行通常的powershell部署命令时:

New-AzureRmResourceGroupDeployment -ResourceGroupName MyResources -TemplateFile .\extensionexample.json -vmName MyVmName01

结果我失败了,并且:

“消息”:“VM在处理扩展时报告了故障 “RunDateTimeScript”。错误消息:\“已完成执行命令\”