Facing issue in securing sever-side app

Hi, We are working on building a server-side app to generate content for the iframe (to be shown in the sidebar). We have created a sample application so far. Now, We are trying to secure the app following this article - https://developer.zendesk.com/documentation/apps/build-an-app/building-a-server-side-app/bonus-part-secure-the-app/.

This tutorial explains adding a security feature in which Zendesk includes JWT token in the request for the initial page.

Problem

If we set `signedUrls` to true to make the app secure, it shows the following error message on the sidebar.

manifest.json

{
  "name": "My Cat App",
  "author": {
    "name": "Zendesk",
    "email": "support@zendesk.com",
    "url": "https://support.zendesk.com"
  },
  "defaultLocale": "en",
  "private": true,
  "signedUrls": true,
  "location": {
    "support": {
      "ticket_sidebar": {
        "url": "https://neeraj.free.beeceptor.com/zendesk",
        "flexible": true,
        "size": {
          "height": "375px"
        }
      }
    }
  },
  "domainWhitelist": ["thecatapi.com", "api.thecatapi.com"],
  "version": "1.0",
  "frameworkVersion": "2.0"
} 

How do we resolve this issue? Please let us know if you need any other information.