Well as you’ve got the code successfully set up to subscribe to topics, and API Gateway to route Twitch’s notifications to your Lambda function, you could try to write your own notification handler.
The EventSub docs include how to handle notifications and process them EventSub | Twitch Developers, and while I’m not sure why the tutorial function isn’t working for you it should give you and idea of how you can handle notifications yourself as it is intended as just a starting point to show how the various services work together.
You’ve already seen how you can console.log the headers and body so you can use that information to try write a verification function yourself, or even just for testing skip that function entirely just to test responding with the challenge and successfully having a subscription enabled (although in production, please properly verify messages to ensure they are from Twitch.