please check that your Lambda function response matches the expected format: https://docs.aws.amazon.com/lex/latest/dg/lambda-input-response-format.html#using-lambda-response-format.
You can also look at the following gist that provides a sample implementation and some utility functions https://gist.github.com/alexcasalboni/3ea2d8dda11c6b73bbf98adf2dd6a214
Feel free to post a sample response in case you still cannot solve the issue, taking care of removing any sensitive information.
lex / lambda order flowers blueprints give invalid lambda response errorasked 8 months ago
integrate AWS Lex V1 with website using pythonasked 5 months ago
Having issue while sending lambda response to Lex v2. Error : "Invalid Lambda Response: Can not construct instance of IntentResponse"asked 9 months ago
JSON input in Amazon Lex test consoleasked 10 months ago
Amazon lex Chat Bot testAccepted Answerasked 2 months ago
V2 Lex Order Flowers Lambda error
An error has occurred: The server encountered an error processing the Lambda responseasked 3 months ago
Lex Internal Server Error (deployed from Terraform)asked a year ago
[Issue] Lambda LexEvent returns "null" values from Lex v2 Requestasked 10 months ago
V2 Lex OrderFlowers Blueprint Lambda Error