POST event on outlook calendar using node.js : error InvalidAuthenticationToken - node.js

Question

I am using this code to authenticate on Outlook Calendar (Oauth2) and then post an event to my calendar. I'm getting an error that after my first authentication the token is expired so O can't post on the calendar. If someone can help to find the error in my code :)
var express = require('express');
var app = express();
var bodyParser = require('body-parser');
var cookieParser = require('cookie-parser');
var session = require('express-session');
var moment = require('moment');
var querystring = require('querystring');
var outlook = require('node-outlook');
var microsoftGraph = require("#microsoft/microsoft-graph-client");
var currentToken;
var authHelper = require('./authHelper');
// 3- Configure express
// Set up rendering of static files
app.use(express.static('static'));
// Need JSON body parser for most API responses
app.use(bodyParser.json());
// Set up cookies and sessions to save tokens
app.use(cookieParser());
app.use(session(
{ secret: '************',
resave: false,
saveUninitialized: false
}));
// 4- Home page
app.get('/', function(req, res) {
res.write('<a id="signin-button" class="ms-Button ms-Button--primary"
href="'+authHelper.getAuthUrl()+ '"><span class="ms-Button-label">Click here
to sign in</span></a>');
res.end();
});
app.get('/authorize', function(req, res) {
var authCode = req.query.code;
if (authCode) {
console.log('');
console.log('Retrieved auth code in /authorize: ' + authCode);
authHelper.getTokenFromCode(authCode, tokenReceived, req, res);
}
else {
// 5- redirect to home
console.log('/authorize called without a code parameter, redirecting to
login');
res.redirect('/');
}
});
// 6- if it can't take the token
function tokenReceived(req, res, error, token) {
if (error) {
console.log('ERROR getting token:' + error);
res.send('ERROR getting token: ' + error);
}
else {
// 7- save tokens in session
req.session.access_token = token.token.access_token;
req.session.refresh_token = token.token.refresh_token;
req.session.email = authHelper.getEmailFromIdToken(token.token.id_token);
res.redirect('/logincomplete');
}
}
// 8- login complete
app.get('/logincomplete', function(req, res) {
var access_token = req.session.access_token;
var refresh_token = req.session.access_token;
var email = req.session.email;
currentToken = access_token;
var http = require("https");
var options = {
"method": "POST",
"hostname": "graph.microsoft.com",
"port": null,
"path": "/v1.0/me/events",
"headers": {
"authorization": "Bearer 'access_token'",
"content-type": "application/json",
"cache-control": "no-cache",
}
};
var req = http.request(options, function (res) {
var chunks = [];
res.on("data", function (chunk) {
chunks.push(chunk);
});
res.on("end", function () {
var body = Buffer.concat(chunks);
console.log(body.toString());
});
});
req.write("{\n \"subject\": \"My M\",\n \"start\": {\n \"dateTime\":
\"2017-11-30T15:34:40.942Z\",\n \"timeZone\": \"UTC\"\n },\n \"end\":
{\n \"dateTime\": \"2017-12-07T15:34:40.942Z\",\n \"timeZone\":
\"UTC\"\n }\n}");
req.end();
});
// Start the server
var server = app.listen(3000, function() {
var host = server.address().address;
var port = server.address().port;
console.log('Example app listening at http://%s:%s', host, port);
});