How to access Google sheet with C# and a service account

GoogleSheetsCsharp

var certificate = new X509Certificate2(@"c:\Diamto Test Everything Project.p12", "notasecret", X509KeyStorageFlags.Exportable);

const string user = "XXX@developer.gserviceaccount.com";

var serviceAccountCredentialInitializer = new ServiceAccountCredential.Initializer(user)  {Scopes = new[] { "https://spreadsheets.google.com/feeds" }
               }.FromCertificate(certificate);

var credential = new ServiceAccountCredential(serviceAccountCredentialInitializer);

if (!credential.RequestAccessTokenAsync(System.Threading.CancellationToken.None).Result)
    throw new InvalidOperationException("Access token request failed.");

var requestFactory = new GDataRequestFactory(null);
requestFactory.CustomHeaders.Add("Authorization: Bearer " + credential.Token.AccessToken);

var service = new SpreadsheetsService(null) { RequestFactory = requestFactory };

// Instantiate a SpreadsheetQuery object to retrieve spreadsheets.
SpreadsheetQuery query = new SpreadsheetQuery();

// Make a request to the API and get all spreadsheets.
SpreadsheetFeed feed = service.Query(query);

if (feed.Entries.Count == 0)
  {
  Console.WriteLine("There are no sheets");
  }

// Iterate through all of the spreadsheets returned
foreach (SpreadsheetEntry sheet in feed.Entries)
  {
  // Print the title of this spreadsheet to the screen
  Console.WriteLine(sheet.Title.Text);
  
  // Make a request to the API to fetch information about all
  // worksheets in the spreadsheet.
  WorksheetFeed wsFeed = sheet.Worksheets;

  // Iterate through each worksheet in the spreadsheet.
  foreach (WorksheetEntry entry in wsFeed.Entries)
      {
      // Get the worksheet's title, row count, and column count.
      string title = entry.Title.Text;
      var rowCount = entry.Rows;
      var colCount = entry.Cols;

     // Print the fetched information to the screen for this worksheet.
      Console.WriteLine(title + "- rows:" + rowCount + " cols: " + colCount);

     // Create a local representation of the new worksheet.
     WorksheetEntry worksheet = new WorksheetEntry();
     worksheet.Title.Text = "New Worksheet";
     worksheet.Cols = 10;
     worksheet.Rows = 20;

    // Send the local representation of the worksheet to the API for
    // creation.  The URL to use here is the worksheet feed URL of our
    // spreadsheet.
    WorksheetFeed NewwsFeed = sheet.Worksheets;
    service.Insert(NewwsFeed, worksheet);
    }
  }

Hi, I've got error :{"Error:"invalid_grant", Description:"", Uri:"""}
related to the instruction: RequestAccessTokenAsync

any help?