Problem using SSRS 2016 webservice

I am doing some automation with SQL 2016 Reporting Services by accessing its Management web service.

Here is my code:

 private Policy[] GetItemPolicy(string itemPath = "/", Boolean InheritParent = true)
        {
            ReportingService2010 rs = new ReportingService2010();
            rs.Credentials = System.Net.CredentialCache.DefaultCredentials;
            rs.Url = url;
            try
            {
                return rs.GetPolicies(itemPath, out InheritParent);
            }
            catch(Exception ex)
            {
                log(ex.Message);
            }
            return rs.GetPolicies(itemPath, out InheritParent);
        }

The ex.Message is The request failed with HTTP status 403: Forbidden

I have added a machine account where it's running to Report Portal. Any idea what the problem might be?

Both my code (IIS) and SSRS SQL Server are running on the same box.

While I was developing this solution I was using a distributed architecture where application server and SSRS server were on different machines.

It worked fine there. Is there any restriction using the same approach on a single machine?

Thanks