I am getting the following error when i tried to authorize gmail api using service account

"Client is unauthorized to retrieve access tokens using this method"

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();

I am using the p12 key which i got while creating service account.But when i run my console app the following error occurs.Any help would be really appreciated.

Thanks in advance !