question

I want to update the display name but my cloud script code always fails. I want that a player can only update his display name once(when the display name is not configured), therefore I wrote this code: if (DisplayName != null).

Why is my cloud script code not working?

Raw event JSON
{
    "EventName": "player_executed_cloudscript",
    "Source": "CloudScript",
    "FunctionName": "UpdateDisplayname",
    "CloudScriptExecutionResult": {
        "FunctionName": "UpdateDisplayname",
        "Revision": 16,
        "FunctionResult": null,
        "FunctionResultTooLarge": null,
        "Logs": [],
        "LogsTooLarge": null,
        "ExecutionTimeSeconds": 0.00042869999999999996,
        "ProcessorTimeSeconds": 0,
        "MemoryConsumedBytes": 24552,
        "APIRequestsIssued": 0,
        "HttpRequestsIssued": 0,
        "Error": {
            "Error": "JavascriptException",
            "Message": "JavascriptException",
            "StackTrace": "ReferenceError: DisplayName is not defined\n    at handlers.UpdateDisplayname (BFD0A-main.js:4:5)"
        }
    },
    "EventNamespace": "com.playfab",
    "EntityType": "player",
    "TitleId": "BFD0A",
    "EntityId": "2948776B356793A8",
    "EventId": "1b31881167554232b591fc39187d67eb",
    "SourceType": "BackEnd",
    "Timestamp": "2019-09-16T15:23:27.1164935Z",
    "History": null,
    "CustomTags": null,
    "Reserved": null,
    "PlayFabEnvironment": {
        "Vertical": "master",
        "Cloud": "main",
        "Application": "logicserver",
        "Commit": "91b7bbc"
    }
}

My cloud script code:

handlers.UpdateDisplayname = function (args, context)
{
    var NewDisplayname = args.DesiredDisplayname;
    if (DisplayName != null)
      server.UpdateUserTitleDisplayName({PlayFabID: currentPlayerId, DisplayName: NewDisplayname});
}

Client code:

private async Task UpdateDisplayname()
{
    var result = await PlayFabClientAPI.ExecuteCloudScriptAsync(new ExecuteCloudScriptRequest()
    {
        FunctionName = "UpdateDisplayname",
        FunctionParameter = new { DesiredDisplayname = "Mynewdisplayname" },
        GeneratePlayStreamEvent = true
    });

    if (result.Error != null)
        Console.WriteLine(result.Error.Error.ToString());
    else
        Console.WriteLine("Your new displayname: " + "Mynewdisplayname");
}