Welcome to your new Real-Time LCA Community - so happy to see you here! 
Please feel free to introduce yourself to the Community here:
Really cool that you want to leverage the data in other applications!
To answer your question, the error might occur since you need to choose the production environment in the top left corner of the Postman app, before you execute the Authenticate user POST request.
It looks something like this:
This sets the urls to the production environment for the different requests in the Postman Collection.
Hope it works out for you!
/Martin