I have no experience with Newman, or trying to do anything of this sort in Postman as it’s not really fit for this purpose, but from a technical point of view the verification failed error indicates that either your callback URL is not internet accessible and reachable by Twitch, or it doesn’t correctly return the challenge sent by Twitch.
You should be able to test this yourself by sending a POST request to your callback URL with a challenge in the format shown in the docs, and see what your server responds with.
Also, the secret you’re providing should NOT be your client secret, that’s an entirely different thing. The secret you provide when subscribe to EventSub is a secret you generate yourself and pass to Twitch for them to sign the notification with, and you verify that signature on receiving a notification.