How to solve ' 502 Bad Gateway ' nginx ??

If I run on my droplet ubuntu 20.04 with nginx a simple test nodejs server, it works:

const http = require('http');

const hostname = 'localhost';
const port = 3000;

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello World!\n');
});

server.listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);
});

but when I try to update it with my more complex node server I get back ’ 502 Bad Gateway '.

error.log:

2021/03/29 10:49:21 [error] 1430#1430: *1352 connect() failed (111: Connection refused) while connecting to upstream, client: 2.45.152.97, server: ferramentadenicolo.com, request: "GET /favicon.ico HTTP/1.1", upstream: "http://127.0.0.1:3000/favicon.ico", host: "www.ferramentadenicolo.com"

Trying to understand what give me back this error, I noticed that just adding to the simple test a string like:

const api = require('./routes/api');

it returns me the 502

Some advice please? I’m really new about deploying a server and I don’t know how to proceed. THX