An entry point cannot be marked with the 'async' modifier

Starting from C# 7.1 there are 4 new signatures for Main method which allow to make it async(Source, Source 2, Source 3):

public static Task Main();
public static Task<int> Main();
public static Task Main(string[] args);
public static Task<int> Main(string[] args);

You can mark your Main method with async keyword and use await inside Main:

static async Task Main(string[] args)
{
    Task<string> getWebPageTask = GetWebPageAsync("http://msdn.microsoft.com");

    Debug.WriteLine("In startButton_Click before await");
    string webText = await getWebPageTask;
    Debug.WriteLine("Characters received: " + webText.Length.ToString()); 
}

C# 7.1 is available in Visual Studio 2017 15.3.

The error message is exactly right: the Main() method cannot be async, because when Main() returns, the application usually ends.

If you want to make a console application that uses async, a simple solution is to create an async version of Main() and synchronously Wait() on that from the real Main():

static void Main()
{
    MainAsync().Wait();
}

static async Task MainAsync()
{
    // your async code here
}

This is one of the rare cases where mixing await and Wait() is a good idea, you shouldn't usually do that.

Update: Async Main is supported in C# 7.1.