I can tell you that your base API URL matches what I have working in my Postman sandbox to our Jira Cloud instance. I can also tell you that getting the authentication working was an exercise in patience.
I'd string together that base URL and put it right into your browser. If you get results there, then you know the syntax is right and the issue is something in your JSON path or your authentication steps. Take the results you get, run those through something like JSON Path Finder to validate the path data there. If that's also right, then you're down to figuring out how to pass the authentication variables.