Export PDF from paginated report in c#

Hi All,

I am trying to export a paginated report into pdf from c# application. Below is my code:

private async Task<string> PostExportRequest(Guid reportId, Guid groupId)
{
try
{
var paginatedReportExportConfiguration = new PaginatedReportExportConfiguration()
{
FormatSettings = new Dictionary<string, string>() { { "PageHeight", "14in" }, { "PageWidth", "8.5in" }, { "StartPage", "1" }, { "EndPage", "1" } }
};

var exportRequest = new ExportReportRequest { Format = FileFormat.PDF, PaginatedReportConfiguration = paginatedReportExportConfiguration };

var _apiUrl = new Uri("https://app.powerbi.com/groups/<groupId>/rdlreports/<reportId>/ExportTo");
var handler = new System.Net.Http.HttpClientHandler() { UseDefaultCredentials = false };
string clientID = "xxx-xxx-xxx-xxx-xxx-xxxx";
string secret = "xxx-xxx-xxx-xxx-xxx-xxxx";
var credentials = new Microsoft.IdentityModel.Clients.ActiveDirectory.ClientCredential(clientID, secret);
var AuthorityUrl = "https://login.microsoftonline.com/consumers/oauth2/v2.0/<my application tenentid>";
var authenticationContext = new AuthenticationContext(AuthorityUrl);
var authenticationResult = await authenticationContext.AcquireTokenAsync("https://analysis.windows.net/powerbi/api", credentials).ConfigureAwait(false);
var tokenCredentials = new TokenCredentials(authenticationResult.AccessToken, "Bearer");
var pbClient = new PowerBIClient(_apiUrl, tokenCredentials);
var export = await pbClient.Reports.ExportToFileInGroupAsync(groupId, reportId, exportRequest);
return export.Id;
}
catch (Exception ex){} return null;

}

Whenever i try to run the code I get auth token correctly, however ExportToFileInGroupAsync gives me an error of below:

{"Operation returned an invalid status code 'InternalServerError'"}

Does anybody know what I am doing wrong here?

Appreciate your help in advance.

Thanks
Lloyd