Newbie100%: 502 error and [YOUR_SERVER_FQDN]?

[gcplaw]

Background
I set up Ubuntu 18.04.1 LTS on my cloud server and it gives me a public ip(lets say 2.2.2.2) and an intranet IP.

My issue
I dont have a doman name so I set my FDQN with my public ip.
When i access my public ip, it returns 502 bad gateway error with colored favicon of huggin.
I tried troubleshoot with 'sudo bundle exec rake production:check' and it returns green ok everything is fine.

What should I do now? THANKS!

[gcplaw]

When I click @, you four guys pop up... sorry to bother you guys like this but I really like to know how to solve the issue... apologize again for any inconveniences.....
@brynary @amoeba @necrodome @raucao

[gcplaw]

What I tried: ... too many bad returns...

1. Troubleshooting - sudo bundle exec rake production:check
   Return: [ OK ] Everything is fine

2. Troubleshooting - Nginx error log /var/log/nginx/huginn_error.log
   Return: connect() to unix:/home/huginn/huginn/tmp/sockets/unicorn.socket failed
   BUT i did follow the guide to set the Procfile as below:
   

3. Troubleshooting - Unicorn log /home/huginn/huginn/log/unicorn.log
   2.1 my unicorn.rb as below:
   

2.2 Gem dependencies not all installed, wonder why as below

[virtadpt]

Nginx is trying to connect to the socket but can't.

ls -alF /home/huginn/huginn/tmp/sockets/unicorn.socket

What comes back?

[gcplaw]

Nginx is trying to connect to the socket but can't.

ls -alF /home/huginn/huginn/tmp/sockets/unicorn.socket

What comes back?

ls: cannot access '/home/huginn/huginn/tmp/sockets/unicorn.socket': No such file or directory

@virtadpt no such file... and thank you for pick up my issue!

[virtadpt]

Yep, that's your problem. No unicorn.socket, nothing for Nginx to connect to. What happens when you try to start up the Unicorn server?

rake production:export