Hi Kate,
Thank you for reaching out to us. We're glad to help you.
As far I can see, there are no errors on the Mendix side.
The most obvious reason probably is that the attribute InvocationType in the InvokeFunctionRequest is set to "EVENT".
This means that AWS will invoke the function asynchronously, so it will not return the payload directly with the invocation of the function.
See for more info on this the AWS Lambda API documentation: https://docs.aws.amazon.com/lambda/latest/api/API_Invoke.html#API_Invoke_RequestSyntax
Does this help you?
Kind Regards on behalf of the Mendix Strategic Alliance Development team,
Emiel