You can test with either JWT or OAuth in Postman. I have a feeling this is related more to your environment. I’d recommend checking out this stack overflow post (it’s a little older, but still links to the Relevant Postman docs):
In regards to OAuth, if you want to test this in Postman, you’ll need to first create an OAuth app in the marketplace and then you can access the Client ID in the credentials section:
I recommend testing with a User-Level OAuth app, if you do so.
I hope this helps!
-Will