Why does this app.js file not "see" my route?

I want to utilize some "best practices" as I build the API you see below, as far as using routes, models and controllers etc.

The code below is something I'm using from "fitbit-node." It works just fine, however. I want to take some of what resonates as that which belongs in a "routes" directory and move it so, when I'm all finished, I can upload it as something that looks and feels like a complete and accurate app.

At least, that's what I'm thinking.

Here's my code:

// initialize the express application
const express = require("express");
const app = express();
const path = require("path");
const bodyParser = require("body-parser");
const mongoose = require("mongoose");
[b]//const authorizeRoutes = require("./routes/authorize");[/b]

app.use(bodyParser.json());
[b]//app.use("/authorize", authorizeRoutes);[/b]

// initialize the Fitbit API client
const FitbitApiClient = require("fitbit-node");
const client = new FitbitApiClient({
	clientId: "CLIENT_ID", //client_id
	clientSecret: "SECRET", //client secret
	apiVersion: '1.2' // 1.2 is the default
});

// redirect the user to the Fitbit authorization page
app.get("/authorize", (req, res) => {
	// request access to the user's activity, heartrate, location, nutrion, profile, settings, sleep, social, and weight scopes
	res.redirect(client.getAuthorizeUrl('activity heartrate location nutrition profile settings sleep social weight', 'http://localhost:3000/callback')); //callback url
});

// handle the callback from the Fitbit authorization flow
app.get("/callback", (req, res) => {
	// exchange the authorization code we just received for an access token
	client.getAccessToken(req.query.code, 'CALLBACK_URL').then(result => { //callback url
		// use the access token to fetch the user's profile information
		client.get("/profile.json", result.access_token)
		.then(results => {
			res.send(results[0]);
			console.log(results[0].user.age);
		}).catch(err => {
			res.status(err.status).send(err);
		});
	}).catch(err => {
		res.status(err.status).send(err);
	});
});

mongoose
    .connect(
        "DATABASE INFO"
    )
    .then(result => {
        app.listen(3000);
    })
    .catch(err => console.log(err));

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You see the commented lines I have in bold? That's my first step in trying to break up the above code into what I'm thinking SHOULD be a collection of references to various routes.

When I un-comment the emboldened lines and use this as my "authorize" route...

const express = require("express");
const app = express();

const router = express.Router();

// initialize the Fitbit API client
const FitbitApiClient = require("fitbit-node");
const client = new FitbitApiClient({
	clientId: "22BD5D", //client_id
	clientSecret: "9677a6cdf63836e26d0617e9ac2c38f8", //client secret
	apiVersion: '1.2' // 1.2 is the default
});

// redirect the user to the Fitbit authorization page
app.get("/authorize", (req, res) => {
	// request access to the user's activity, heartrate, location, nutrion, profile, settings, sleep, social, and weight scopes
	res.redirect(client.getAuthorizeUrl('activity heartrate location nutrition profile settings sleep social weight', 'http://localhost:3000/callback')); //callback url
});

module.exports = router;

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I get a message that says, "Cannot get /authorize."

Why?