I’m also new to Postman myself, but to use it from the browser to access a locally-running server, you need to have the Postman Desktop Agent installed. See the official support article for more help, and this article from the official Postman blog to understand why you need it.
Being a newbie myself, I was confusing the Postman Desktop App with the Postman Desktop Agent. Similar names, but completely different apps (and concepts)!