AWS Cognito Amplify.Auth Password less signIn using CUSTOM_AUTH_WITHOUT_SRP authFlowType gives ServiceException This sign in method is not supported

Question

I am trying Password less signIn using "CUSTOM_AUTH_WITHOUT_SRP" as authFlowType in AWS Cognito Amplify.Auth but it gives me ServiceException with message "This sign in method is not supported. I also tried with "CUSTOM_AUTH" as authFlowType but still it gives me the same error message.

I checked in backend Lamda there it receives my request and sends the success message in response but AWS Cognito sends me Service Exception instead Success as response.

Below is my source code where I am trying to signIn using username.

val auth: KotlinAuthFacade = Amplify.Auth
val options = AWSCognitoAuthSignInOptions.builder()
            .authFlowType(AuthFlowType.CUSTOM_AUTH_WITHOUT_SRP)
            .build()
val result = auth.signIn(username = username, options = options) 

And below is the error I am getting when I try to do SignIn

UnknownException{message=Sign in failed., cause=ServiceException{message=This sign in method is not supported, cause=null, recoverySuggestion=Please consult our docs for supported sign in methods}, recoverySuggestion=Sorry, we don't have a suggested fix for this error yet.}

I checked on Amazon Cognito User Pool App Client there I don't see "CUSTOM_AUTH_WITHOUT_SRP" option to select.

I am stuck here, Any help is greatly appreciated.