Did you ever figure this out? Hitting same issue.
I would assume it's something like: response.errorCode (where "response" is the name of the returned object). This does not work just saying what I think it should be.
Response mappings start with a dot-property of the response object. If the outermost response object is an array, you can't dereference it this way.
If you're forced to use an endpoint like this I would gateway it through something like AWS APIGW and wrap it in another object.
Responses like this aren't typically made for webhook environments.
I was running into the same issue. However I had 2 differences but perhaps this helps you in the right direction.
Firstly I only cared about the first response in the array so that makes it easier but if you are expecting multiple ones you would just need more fields to store the information in.
For the mapping. I didn't have an array name - I know weird but as you I had to work with what I received.
Here is the array response message:
The [ ] means it is an array otherwise these wouldn't be there.
I was able to map the sender field using it using: