Async method Firebase

I'm trying to make my method wait for the execution of the GetValueAsync method.

public async Task DownloadTopScoreAsync()
{
    await reference.GetValueAsync().ContinueWith(task =>
    {
        if (task.IsFaulted)
        {
            Debug.Log("error");
        }
        else if (task.IsCompleted)
        {
            DataSnapshot snapshot = task.Result;
            // Do something with snapshot...
            Debug.Log("hit");
        }
    });
    return;
}

I tried everything, I even copied the ready-made code. But it still gives errors.

CS0433 The type 'Task' exists in both 'Unity.Tasks, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' and 'netstandard, Version=2.1.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'

CS0126 An object of a type convertible to 'Task' is required

CS1061 'Task' does not contain a definition for 'GetAwaiter' and no accessible extension method 'GetAwaiter' accepting a first argument of type 'Task' could be found (are you missing a using directive or an assembly reference?)