Connection refused with port 3000

I am following this tutorial for my node application. https://www.digitalocean.com/community/tutorials/how-to-set-up-a-node-js-application-for-production-on-ubuntu-18-04 Actually I want to use root folder for angular and sub directory ‘api’ for node js API. So I’m trying to connect port 3000 for node js app. But I am getting connection refused message. This port in not in listen state. So I have followed this link. https://askubuntu.com/questions/932557/how-to-listen-new-port-ubuntu-server-from-command-line /etc/systemd/system/listen-3000.service

But still I have the same issue. My /etc/nginx/sites-available/default is,

server {
	listen 80 default_server;
	listen [::]:80 default_server;


	root /var/www/html;

	# Add index.php to the list if you are using PHP
	index index.html index.htm index.ts;

	server_name app.mydomain.com;
	return 301 https://app.mydomain.com$request_uri;

	location / {	
		try_files $uri $uri/ =404;
	}

	location /api/ {
		proxy_pass http://localhost:3000;
	        proxy_http_version 1.1;
	        proxy_set_header Upgrade $http_upgrade;
	        proxy_set_header Connection 'upgrade';
	        proxy_set_header Host $host;
	        proxy_cache_bypass $http_upgrade;
	}

}