I used valid paymentId: PAY-6J516405S23367429LP6IYLQ and it was working as expected. I got the details about PAY-6J516405S23367429LP6IYLQ payment.
I changed the last sign in paymentId from Q to R to see how an error looks like when invalid paymentId is used. But I didn't get the error. The returned JSON was mostly the same like for PAY-6J516405S23367429LP6IYLQ. The only one difference was paymentId in lines: