how to run a node.js file forever?

Question

src.js:

const express = require('express')
const app = express()
const port = 3000

app.get('/', (req, res) => {
  res.send('Hello World!')
})

app.listen(port, () => {
  console.log(`Example app listening on port ${port}`)
})

I made this for my website, i want this file to run forever, but when i try to install pm2 by npm i pm2 -g it throws this error:

npm ERR! code ENOENT

npm ERR! syscall mkdir

npm ERR! path /usr/lib/node_modules/pm2

npm ERR! errno -2

npm ERR! enoent ENOENT: no such file or directory, mkdir /usr/lib/node_modules/pm2

npm ERR! enoent This is related to npm not being able to find a file.

npm ERR! enoent

how to fix it?

and my second question if i run it with node src.js it listens on mywebdomain:3000, i want it to listen on mywebdomain/. how do i do that?