404 Error when making a POST request using fetch() - mysql

Question

I am following this tutorial from Hakernoon to set up Node.js with MySQL using my machine as local server.
I am using a few different tools, but they don't seem to be a problem. I'm using:
Ubuntu 16.04
Node.js
MySQL Workbench (He uses Homebrew because his is a Mac)
MySQL
knex.js
Express
*(Ubuntu and Workbench are the only differences)
Summarizing,the whole tutorial works well except when I reach the part of 'Login'. Here, we build a form to enter the user name and password making a POST requests. Then, if everything is OK, it returns a 200 status, else shows a message saying the login failed. And here is the problems. Every time I try to log in (even when being sure the credentials are correct and knowing that that user is stored in the database) it tells me that the login fails.
Checking on the browser inspect tool, it shows a Fail to load resources: the server responded with a status of 404 (Not found). It also points out that the problems is in the fetch() function I use in my post function to make the POST requests.
I checked the documentation on fetch() but is does not clarify much. Also I tried to contact the author of the tutorial and I got no answer. What am I doing wrong with fetch to give me the 404 status?
Also, every time I create a new user (also a POST request), it does it with no problem and I can see it in the database in MySQLWorkbench.
This is the file (app.js) with the code to create users and login:
const CreateUser = document.querySelector('.CreateUser')
CreateUser.addEventListener('submit', (e) => {
e.preventDefault()
const username = CreateUser.querySelector('.username').value
const password = CreateUser.querySelector('.password').value
post('/createUser', { username, password })
})
const Login = document.querySelector('.Login')
Login.addEventListener('submit', (e) => {
e.preventDefault();
const username = Login.querySelector('.username').value
const password = Login.querySelector('.password').value
post('/login', {username, password})
.then(({status}) => {
if(status === 200)
alert('login success')
else
alert('login failed')
})
})
function post (path, data) {
return window.fetch(path, { <========= Here is where the browser shows the problem
method: 'POST',
headers: {
'Accept': 'application/json',
'Content-Type': 'application/json'
},
body: JSON.stringify(data)
})
}
***I am not posting the rest of the code to not make it too long, if more info is needed, let me know). Thanks!