Power BI Embedded API ExportToFile returns "Forbidden" error

I'm trying to do an ExportToFile on a report I have displayed in my app using the Power BI Embedded API.  I am using the same code to get token credentials and generate a Power BI Embedded API client object that is working when I display my report, but when I try to call Clent.Reports.ExportToFIleInGroupsAsync, it throws a "Forbidden" exception.

var exportRequest = new ExportReportRequest
{
Format = format,
PowerBIReportConfiguration = powerBIReportExportConfiguration
};

// Get token credentials for user
var getCredentialsResult = await GetTokenCredentials();
if (!getCredentialsResult)
{
// The error message set in GetTokenCredentials
return null;
}

var Client = new PowerBIClient(new Uri(ApiUrl), m_tokenCredentials);
var export = await Client.Reports.ExportToFileInGroupAsync(groupId, reportId, exportRequest);

Any thoughts on what I'm doing wrong?

-- Tony