Error: No access, refresh token or API key is set when accessing users data

I'm very new to the concept of API and nodejs. I have a task to create a simple agent that accesses users events on google calendar to remind him on his upcoming event, but I have been struggling with this error "Error: No access, refresh token or API key is set". I have followed a guide on how to authorize and retrieve the access token but it seems to not work. Here is my main.js file:

const google = require('googleapis').google;
const dfff = require('dialogflow-fulfillment')

const app = express();

const googleConfig = {
  clientId: 'xxxxxxxxxxxx', 
  clientSecret: 'xxxxxxxxxxxx', 
  redirect: 'https://d5242f9e0cb4.ngrok.io' 
};

function createConnection() {
  return new google.auth.OAuth2(
    googleConfig.clientId,
    googleConfig.clientSecret,
    googleConfig.redirect
  );
}

const defaultScope = [
   'https://www.googleapis.com/auth/calendar.readonly'
];

function getConnectionUrl(auth) {
  return auth.generateAuthUrl({
    access_type: 'offline',
    prompt: 'consent', // access type and approval prompt will force a new refresh token to be made each time signs in
    scope: defaultScope
  });
}

function urlGoogle() {
  const auth = createConnection(); // this is from previous step
  const url = getConnectionUrl(auth);
  return url;
}
// Setting up EJS Views
app.set('view engine', 'ejs');
app.set('views', __dirname);

app.get('/', function (req, res) {

    const url = urlGoogle()
    return res.render("index", { loginLink: url });

  });
 
app.get('/auth_callback', function (req, res) {
    // Create an OAuth2 client object from the credentials in our config file
    const OAuth = createConnection()
    //if (req.query.error) {
      // The user did not give us permission.
      //return res.redirect('/error');
    //} else {
      const {tokens} = OAuth.getToken(code)
      OAuth.setCredentials({
        access_token: tokens.access_token,
        refresh_token: tokens.refresh_token,
        expiry_date: (new Date()).getTime() + (1000 * 60 * 60 * 24 * 7)
      });      
        return res.redirect('/');
      //});
    //}
  });

  app.post('/', express.json(),(req,res)=>{
    
    const calendar = google.calendar({version: 'v3' , auth:createConnection()});
    
    const agent = new dfff.WebhookClient({
        request : req,
        response : res
      })
      function welcome(agent){
        agent.add("Hi, I'm Rem. Please click on remind me button if you want to be reminded on your upcoming events!")
      }
    
      function listEvents(agent){ 
        return calendar.events.list({
        'calendarId': 'primary',
        'auth':createConnection(),
        'timeMin': (new Date()).toISOString(),
        'showDeleted': false,
        'singleEvents': true,
        'maxResults': 1,
        'singleEvents': true,
        'orderBy': 'startTime'
     }).then((err,response)=> {
        let events = response.result.items;
        if (events.length !== 0) {
            var event = events[i];
            var when = event.start.dateTime;
            if (!when) {
              when = event.start.date;
            }
              agent.add('Be ready for '+ event.summary + ' event '+ 'at ' + when )
          }
        else {
              agent.add('You dont have any upcoming events.');
        }
      });   
         
      }
    
      let intenMap = new Map()
      intenMap.set('Default_Welcome_Intent',welcome)
      intenMap.set('Remind_me',listEvents)

      agent.handleRequest(intenMap)
   
  });

  // Listen on the port defined in the config file
app.listen("3002", function () {
    console.log(`Listening on port 3002`);
  });

What could I be missing??