How to remove error "Sorry, I dont know that one."

Making skills for my favorite avenger, where customer can speak ironman or captain america and skill will reply. but not working.

check out this

const LaunchRequest_Handler = {

canHandle(handlerInput) {

const request = handlerInput.requestEnvelope.request;

return request.type === 'LaunchRequest';

},

handle(handlerInput) {

const responseBuilder = handlerInput.responseBuilder;

let say = 'hello!!! You are here for information about your favorite avenger, Nice to see you here. Which Avenger is your favorite?';

let skillTitle = capitalize(invocationName);

return responseBuilder

.speak(say)

.reprompt('try again, ' + say)

.withStandardCard('Welcome!',

'Hello!\nThis is a card for your skill, ' + skillTitle,

welcomeCardImg.smallImageUrl, welcomeCardImg.largeImageUrl)

.getResponse();

},