Troubleshooting AWS API Gateway Proxy

I have this built an api using express js that I put on aws lambda. I am using aws api gateway to host the api, and so far it has been working so many time.

app.use('/user', userRoute);
export const handler = serverless(app);

Until today, when I wanted to deploy new api, I noticed slightly changes in the API Gateway interface and steps. I set up proxy resource as usual, so that I could use routes I set up in express js app. But once I deployed and tested on postman, it returns

{
    "message": "Internal server error"
}

I noticed that my previous api has been working fine except for this one I recently uploaded. Any Idea how to fix this? thank you.