Integrating PowerBI REST API's with service principal/app registration

[CompactGreen]

Hi,

I am trying to use these specific Power BI REST API's :

   1) Export to File (PNG/PDF)

   2) Get Report

   3) Export Report

At the moment I am able to get the content of a report (screenshot) in its desired format and use the other API's by using an access token which is retrieved using my own credentials (username & password). However, I want to use a service principal/app registration. I am able to get an access token using the client ID and client Secret of the SP/App registration however when passing this to the Power BI REST API I get a return message: 

API is not accessible for application 

I have made sure all the API permission for PowerBI is set under the App Registration already.

Has anyone come across the problem before?