The interface is showed properly (See the attached screenshot). The invoice.js file is compiled as suggested in the lecture. (text is attached below). I think the point is that the route is not specified properly, but I followed literally all the instructions in the video.
PATH OF THE invoice.jade FILE: D:\LNApp\views
PATH OF THE app.js FILE: D:\LNApp
PATH OF THE invoice.js FILE: D:\LNApp\routes
PATH OF THE index.js FILE: D:\LNApp\routes
CODE OF THE INDEX.JADE FILE
h1 Welcome to Lightning Payment!
p This is a payment service for Lightning. Very welcome!
div
h2 submit Payments
form(method=‘post’, action=’/invoice’)
p
label Amount
input(type=‘number’, name=‘Amount’)
p
input(type=‘submit’, name=‘submit’)
CODE OF THE INVOICE.JS FILE
var express = require(‘express’);
var router = express.Router();
const BTCPAY_PRIV_KEY = “4c6a32116362b1c9dbece16785d0c93ac5bab6e1b75717582269aca3671a1c2d”;
const BTCPAY_MERCHANT_KEY = “BuxgfRGbe2XHN1LnBi1SJv7RJoiWxYf7uYPJ7je7nebb”;
// Initialize the client
const btcpay = require(‘btcpay’)
const keypair = btcpay.crypto.load_keypair(new Buffer.from(BTCPAY_PRIV_KEY, ‘hex’));
const client = new btcpay.BTCPayClient(‘https://lightning.filipmartinsson.com’, keypair, {merchant: BTCPAY_MERCHANT_KEY})
/* get & verify invoice. */
router.get(’/:id’, async function(req, res, next) {
});
/* Create invoice. */
router.post(’/’, function(req, res, next) {
var dollarAmount = req.body.amount;
console.log(dollarAmount);
//Create invoice
client.create_invoice({price: dollarAmount, currency: “USD”})
.then(function(invoice){
console.log(invoice);
res.render(“invoice”, {invoiceId: invoice.id})
})
.catch(err => console.log(err));
//Display
//What happens after
});
module.exports = router;
CODE OF THE APP.JS FILE
var createError = require(‘http-errors’);
var express = require(‘express’);
var path = require(‘path’);
var cookieParser = require(‘cookie-parser’);
var logger = require(‘morgan’);
var indexRouter = require(’./routes/index’);
var invoiceRouter = require(’./routes/invoice’);
var app = express();
// view engine setup
app.set(‘views’, path.join(__dirname, ‘views’));
app.set(‘view engine’, ‘jade’);
app.use(logger(‘dev’));
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, ‘public’)));
app.use(’/’, indexRouter);
app.use(’/invoice’, invoiceRouter);
// catch 404 and forward to error handler
app.use(function(req, res, next) {
next(createError(404));
});
// error handler
app.use(function(err, req, res, next) {
// set locals, only providing error in development
res.locals.message = err.message;
res.locals.error = req.app.get(‘env’) === ‘development’ ? err : {};
// render the error page
res.status(err.status || 500);
res.render(‘error’);
});
module.exports = app;
But anyway, Windows does not allow to copy the text from command prompt and I had to copy the key manually… 
