UserLambdaValidationException : Trying to integrate Passwordless Authentication using AWS Cognito and Amplify library in Android App

Based on the error it sounds like there is not a User within Cognito to be referenced or verified rather via the customer challenge. The user needs to register prior to passing a challenge in order to be authenticated. The following exert and additional information can be found in this AWS Blog: https://aws.amazon.com/blogs/security/how-to-implement-password-less-authentication-with-amazon-cognito-and-webauthn/

"Amazon Cognito user pools allow you to build a custom authentication flow that uses AWS Lambda functions to authenticates users based on one or more challenge/response cycles. You can use this flow to implement password-less authentication that is based on custom challenges. To use this flow to implement FIDO authentication, you need to create credentials during the registration phase and reference these credentials in the user’s profile. You can then validate these credentials during the authentication phase in a custom challenge."