Github Probot create-probot-app: 'Cannot POST /' (404) response to webhook request

Testing out the npx create-probot-app; tried with each of the starters and the same problem exists.

I create, build, and run the app, and then configure and install the app on github, I am able to receive webhook events but I'm seeing my local app respond with a 404.

smee receives the event

error: Error: cannot POST / (404)
ERROR (server): Not Found
    Error: Not Found
        at Request.callback (/Users/X/Projects/Y/compiler/githubapp/x-bot/node_modules/superagent/lib/node/index.js:884:15)
        at IncomingMessage.<anonymous> (/Users/X/Projects/Y/compiler/githubapp/x-bot/node_modules/superagent/lib/node/index.js:1127:20)
        at IncomingMessage.emit (events.js:326:22)
        at IncomingMessage.EventEmitter.emit (domain.js:483:12)
        at endReadableNT (_stream_readable.js:1241:12)
        at processTicksAndRejections (internal/process/task_queues.js:84:21)
status: 404

My index.ts:

export = (app: Probot) => {
  app.on("issues.opened", async (context) => {
    const issueComment = context.issue({
      body: "Thanks for opening this issue!",
    });
    await context.octokit.issues.createComment(issueComment);
  });
  // For more information on building apps:
  // https://probot.github.io/docs/

  // To get your app running against GitHub, see:
  // https://probot.github.io/docs/development/
};

I've checked that my environment variables appear to be setup correctly in .env

In sum, github is sending a webhook, smee is receiving it, but my local app is responding with 404: cannot post /

Not able to get the example app to process webhook events due to what appears to be a routing issue. Anyone encountered this before?

Solution

Make sure you don't have any other applications running on Port 3000. Create React App uses 3000 by default and can cause the error you are describing.