问题 客户未经授权使用此方法检索访问令牌Gmail API C＃

当我尝试使用服务帐户授权gmail api时，我收到以下错误

“客户端未经授权使用此方法检索访问令牌”

static async Task MainAsync()
    {

        sstageEntities db = new sstageEntities();
        //UserCredential credential;
        Dictionary<string, string> dictionary = new Dictionary<string, string>();    
String serviceAccountEmail =
"xxx.iam.gserviceaccount.com";

        var certificate = new X509Certificate2(
            AppDomain.CurrentDomain.BaseDirectory +
              "xxx-8c7a4169631a.p12",
            "notasecret",
            X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.Exportable);

        //string userEmail = "user@domainhere.com.au";

        ServiceAccountCredential credential = new ServiceAccountCredential(
            new ServiceAccountCredential.Initializer(serviceAccountEmail)
            {
                User = "xxx@xxx.com",
                Scopes = new[] { "https://mail.google.com/" }
            }.FromCertificate(certificate)
        );


        // Create Gmail API service.
        var gmailService = new GmailService(new BaseClientService.Initializer()
        {
            HttpClientInitializer = credential,
            ApplicationName = ApplicationName,
        });

        // Define parameters of request.

        var emailListRequest = gmailService.Users.Messages.List("xxx@xxx.com");
        emailListRequest.LabelIds = "INBOX";
        emailListRequest.IncludeSpamTrash = true;
        emailListRequest.Q = "from:bpm@xxx.co.in is:unread";



        //Get our emails
        var emailListResponse = await emailListRequest.ExecuteAsync();

我正在使用创建服务帐户时获得的p12键。但是当我运行我的控制台应用程序时，会发生以下错误。任何帮助都将非常感激。

提前致谢 ！