This error indicates that Twitch called your Callback.
And your callback didn’t respond correclty.
So it didn’t complete the setup of the Webhook/EventSub.
You can use the Twitch CLI to help test this.
twitch event verify-subscription follow -F https://myngrokurl/webhooks/callback/ -s rString
Swap rString with your intended secret naturally.
This will help you debug/test your Callback