I saw a similar topic to this one with the same title, but i rekon the issue is a lot different, and i thought it would be better to make a separate question rather than filling up responses from that one. So let’s go:

Full error: 
[error] 1209#1209: *2283 connect() failed (111: Connection refused) while connecting to upstream, client: 178.222.123.190, server: app.multiunitpros.com, request: "GET /api/ HTTP/1.1", upstream:http://127.0.0.1:1337/", host: "app.multiunitpros.com"

After running sudo nginx -t i get this set of messages

nginx: [warn] conflicting server name “app.multiunitpros.com” on 0.0.0.0:80, ignored

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

nginx: configuration file /etc/nginx/nginx.conf test is successful

Accessing this domain with /api gives 502 bad gateway

Error log provides this

[error] 1209#1209: *2283 connect() failed (111: Connection refused) while connecting to upstream, client: 178.222.123.190, server: app.multiunitpros.com, request: “GET /api/ HTTP/1.1”, upstream: “http://127.0.0.1:1337/”, host: “app.multiunitpros.com”

Here is my /etc/nginx/sites-available/app.multiunitpros.com

server {
  server_name app.multiunitpros.com www.app.multiunitpros.com app.multiunitpros.com;
  error_log /var/www/app.multiunitpros.com/logs/app.multiunitpros.com.error.log;
  access_log /var/www/app.multiunitpros.com/logs/app.multiunitpros.com.access.log;
  root /var/www/app.multiunitpros.com/public/dist;
  index index.php index.html;
  large_client_header_buffers 4 16k;

  location /api {
    proxy_pass http://localhost:1337;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_pass_request_headers      on;
    client_max_body_size 100M;
    rewrite ^/api/?(.*) /$1 break;
  }

  location / {
    try_files $uri $uri/ @rewrites;
  }

  location @rewrites {
    rewrite ^((?!/api/).+)$ /index.html last;
  }


    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/app.multiunitpros.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/app.multiunitpros.com/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot


}


server {
    if ($host = www.app.multiunitpros.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot


    if ($host = app.multiunitpros.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot


  listen 80;
  server_name app.multiunitpros.com www.app.multiunitpros.com app.multiunitpros.com;
    return 404; # managed by Certbot
}