Power BI Export to file Error

[Anonymous]

We are using the latest version of the API (V3)
I have copied the code examples from here - 

https://docs.microsoft.com/en-us/power-bi/developer/embedded/export-to

in my 'PostExportRequest' function I have tried the following - 

var Client = GetPowerBiClient();
var export = Client.Reports.ExportToFileAsync(reportId, exportRequest); 
export.Wait(); 
var exportResult = export.Result;

On the wait, I get a 'Forbidden' error

The user accessing this is acting as a Service Principal and is set to have full admin rights on the PowerBI settings.

on this -

var Client = GetPowerBiClient(); var export = Await Client.Reports.ExportToFileAsync(reportId, exportRequest);

This never completes and seems to run forever.