All API routes in Next JS do not return any content on the server but working fine locally

Question

I am building a Next JS app. The Next JS version I am using is ^13.4.19. I have a an API route with the following code (the filepath is src/pages/api/test.ts).

const testRoute = async(req: NextApiRequest, res: NextApiResponse) => {
  res.status(200).json({
    message: "testing",
  });
};

export default testRoute;

When I was on accessing the route locally, it was working as expected. Then after I deploy it to the server (I am using Serverless Stack - SST), it does not return any content. Instead I am just seeing the blank page.

This is the request info

when I access the route in the postman, I am getting 200 status code. But still empty response/ blank page.

I also tried changing the next js config not to generate Etag like this in next.config.js like this.

api: {
    generateEtags: false,
  },
  generateEtags: false,

But it's not working. What is wrong with my code and how can I fix it?