I try to connect from Postman to the WCO APIs and I always have a unauthorized access:
1) Definition of the environnement variables:
2) Inherits the environnement, GET API for the list of groups:
Probably I not use Postman correctly, but I really need to connect...
I suspect that you haven't imported the environment, on the Refinitiv Dev portal along with the postman collection you also have the postman environment under the download section.
You will have to download the environment as well and import it on your Postman and add your API key & Secret and retry the request it should work.
Let me know if you still have difficulty we can jump on a quick call and set this up for you.