Express js render error this.engine is not a function - express

Question

I'm trying to develop a small API using express. Just want to have 2 views, which, in my case, means 2 html files. One accesing as default with "/" and the other with "/lessons", so we got 2 get controls plus another one which handles every other get input.
*Both files are in the "views" folder and their extension is: *.html
I have no problem accessing the "app.get("/lessons", function..." in fact I know I can acces to that because a simple "console.log(..)" command. The problem is that I got the next error when trying to render:
[TypeError: this.engine is not a function].
Could you help me? I can't understand where is the problem or what I'm doing wrong. I believe it's in the rendering function and has something to do with its configuration and the lessons.html file because index.html has no problem using the same approach.
var express = require('express');
var app = express();
var mod = require('./module');
app.use(express.static('public'));
app.use(express.static('views'));
var port = process.env.PORT || 8080;
app.listen(port, function() {
console.log('Node.js listening on port ' + port);
});
app.get("/", function(req, res) {
console.log("Passed through /");
res.render('index.html');
});
app.get("/lessons", function(req, res) {
console.log("passed through lessons");
res.render('lessons.html', function(err, html) {
if(err) {
console.log(err);
}
res.send(html);
});
//I have tried to to use just: res.render('lessons.html');
});
app.get("*", function(req, res) {
var usageReq = false;
var urlPassed = req.url;
urlPassed = urlPassed.substring(1, urlPassed.length); //remove first "/"
var expected = mod.seeIfExpected(urlPassed); //returns url(if url) or num(if number) or na(if it doesn't match any)
mod.processInfo(expected, urlPassed, function(answer) {
if (answer.found == false && answer.jsonRes == true && answer.info != "inserted") {
res.json({
"error": answer.info
});
} else {
if (answer.jsonRes == true) {
res.json({
"long_url": answer.url,
"short_url": answer.id
});
} else { // go to url
var newUrl = "https://" + answer.url;
res.redirect(newUrl);
}
}
});
});