502 Bad Gateway nginx/1.14.0 (Ubuntu)

First I found out that there was no firewall attached to the droplet. Then I attached a firewall rule to it. But, after that I got Error: 502 Bad Gateway nginx/1.14.0 (Ubuntu)

Here are error logs of /var/log/nginx/error.log:

2020/10/30 04:30:33 [error] 966#966: *28 connect() failed (111: Connection refused) while connecting to upstream, client: 71.85.115.26, server: _, request: "POST /editBlackAndWhiteList HTTP/1.1", upstream: "http://127.0.0.1:1338/editBlackAndWhiteList", host: "159.203.49.172"
2020/10/30 04:32:04 [error] 966#966: *30 connect() failed (111: Connection refused) while connecting to upstream, client: 122.117.89.127, server: _, request: "GET / HTTP/1.1", upstream: "http://127.0.0.1:1338/", host: "127.0.0.1"

Also, the result of sudo nginx -t is:

nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
nginx: configuration file /etc/nginx/nginx.conf test is successful

I checked whether nginx is running systemctl status nginx:

● nginx.service - A high performance web server and a reverse proxy serv
   Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor pr
  Drop-In: /etc/systemd/system/nginx.service.d
           └─override.conf
   Active: active (running) since Fri 2020-10-30 03:34:28 UTC; 1h 12min
     Docs: man:nginx(8)
  Process: 964 ExecStartPost=/bin/sleep 0.1 (code=exited, status=0/SUCCE
  Process: 960 ExecStart=/usr/sbin/nginx -g daemon on; master_process on
  Process: 897 ExecStartPre=/usr/sbin/nginx -t -q -g daemon on; master_p
 Main PID: 963 (nginx)
    Tasks: 2 (limit: 2361)
   CGroup: /system.slice/nginx.service
           ├─963 nginx: master process /usr/sbin/nginx -g daemon on; mas
           └─966 nginx: worker process 

Running ps auxf | grep nginx gives:

root     20160  0.0  0.0  14856   984 pts/0    S+   04:48   0:00  |       \_ grep --color=auto nginx
root       963  0.0  0.0 141120  1544 ?        Ss   03:34   0:00 nginx: master process /usr/sbin/nginx -g daemon on; master_process on;
www-data   966  0.0  0.3 143792  6252 ?        S    03:34   0:00  \_ nginx: worker process

Running ufw status gives:

ufw status
Status: active

To                         Action      From
--                         ------      ----
Nginx HTTP                 ALLOW       Anywhere
OpenSSH                    ALLOW       Anywhere
1337/tcp                   ALLOW       Anywhere
3306                       ALLOW       Anywhere
Nginx HTTP (v6)            ALLOW       Anywhere (v6)
OpenSSH (v6)               ALLOW       Anywhere (v6)
1337/tcp (v6)              ALLOW       Anywhere (v6)
3306 (v6)                  ALLOW       Anywhere (v6)

Here is my file /etc/nginx/nginx.conf:

user www-data;
worker_processes auto;
pid /run/nginx.pid;
include /etc/nginx/modules-enabled/*.conf;

events {
        worker_connections 768;
        # multi_accept on;
}

http {

        ##
        # Basic Settings
        ##

        sendfile on;
        tcp_nopush on;
        tcp_nodelay on;
        keepalive_timeout 65;
        types_hash_max_size 2048;
        # server_tokens off;

        # server_names_hash_bucket_size 64;
        # server_name_in_redirect off;

        include /etc/nginx/mime.types;
        default_type application/octet-stream;

        ##
        # SSL Settings
        ##

        ssl_protocols TLSv1 TLSv1.1 TLSv1.2; # Dropping SSLv3, ref: POODLE
        ssl_prefer_server_ciphers on;

        ##
        # Logging Settings
        ##

        access_log /var/log/nginx/access.log;
        error_log /var/log/nginx/error.log;

        ##
        # Gzip Settings
        ##

        gzip on;

        # gzip_vary on;
        # gzip_proxied any;
        # gzip_comp_level 6;
        # gzip_buffers 16 8k;
        # gzip_http_version 1.1;
        # gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;

        ##
        # Virtual Host Configs
        ##

        include /etc/nginx/conf.d/*.conf;
        include /etc/nginx/sites-enabled/*;
}


#mail {
#       # See sample authentication script at:
#       # http://wiki.nginx.org/ImapAuthenticateWithApachePhpScript
#
#       # auth_http localhost/auth.php;
#       # pop3_capabilities "TOP" "USER";
#       # imap_capabilities "IMAP4rev1" "UIDPLUS";
#
#       server {
#               listen     localhost:110;
#               protocol   pop3;
#               proxy      on;
#       }
#
#       server {
#               listen     localhost:143;
#               protocol   imap;
#               proxy      on;
#       }
#}

How can I make my server run? Any help will be appreciated Also, my project is in /var/www/project:

README.md  config/              node_modules/      public/
admin/     ecosystem.config.js  package-lock.json  server.sh*
api/       extensions/          package.json       yarn-error.log
build/     favicon.ico          podcasting.sql     yarn.lock

Running npm start gives:

[2020-10-30T04:53:56.719Z] debug ⛔️ Server wasn't able to start properly.
[2020-10-30T04:53:56.728Z] error TypeError: Cannot convert undefined or null to object
    at Function.values (<anonymous>)
    at Object.load (/var/www/project/node_modules/strapi-database/lib/connector-registry.js:17:39)
    at DatabaseManager.initialize (/var/www/project/node_modules/strapi-database/lib/database-manager.js:32:21)
    at Strapi.load (/var/www/project/node_modules/strapi/lib/Strapi.js:311:19)
    at async Strapi.start (/var/www/project/node_modules/strapi/lib/Strapi.js:190:9)
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! podcasting@0.1.0 start: `strapi start`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the podcasting@0.1.0 start script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /root/.npm/_logs/2020-10-30T04_53_56_766Z-debug.log