As a rookie at Alexa I recently got this error. It was obvious what it was saying - it couldn't find the NPM package "ask-sdk-core", but I could not figure out why it could not find it.
When you build your Lambda function it does not automatically fetch the NPM packages. These have to be placed together with your index.js file in "the node_modules" folder. It should look something like the below:
Important - choose one of the Alexa blueprints when setting up your lambda function!
It is way easier - at least if you are new to Alexa and Lambda - to use a blueprint. This will automatically give you the folders and modules that you need to get started. Please see my post on how to do this.
Any comments? please leave them below :)