Nginx won't run after symlink creation - ruby-on-rails

Question

Nginx runs fine with an empty sites-enabled directory, but then I create the symlink with
sudo ln -s /etc/nginx/sites-available/default /etc/nginx/sites-enabled/,
and restarting nginx fails. I type nginx -T, and the culprit is
2020/06/29 20:24:19 [emerg] 4087#4087: duplicate upstream "appname" in /etc/nginx/sites-available/default:1
, which is clearly caused my symolink, so I'm not sure what I did wrong.
The original problem I'm trying to fix is getting my EC2 instance to serve my API instead of the default "Welcome to nginx!" page.
Deleting the symlink lets nginx run normally. Here is a shortened version of my output of nginx -T when it works:
# configuration file /etc/nginx/nginx.conf:
user root;
worker_processes auto;
pid /run/nginx.pid;
include /etc/nginx/modules-enabled/*.conf;
events {
worker_connections 20000;
# multi_accept on;
}
http {
disable_symlinks off;
sendfile on;
tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 65;
types_hash_max_size 2048;
include /etc/nginx/mime.types;
default_type application/octet-stream;
...
access_log /var/log/nginx/access.log;
error_log /var/log/nginx/error.log;
include /etc/nginx/conf.d/*.conf;
include /etc/nginx/sites-enabled/*;
include /etc/nginx/sites-available/*;
...
}
# configuration file /etc/nginx/sites-available/default:
upstream appname {
# Path to Puma SOCK file, as defined previously
server unix:/home/ubuntu/appname/shared/sockets/puma.sock fail_timeout=0;
}
server {
listen 80;
listen [::]:80 ipv6only=on default_server;
# server_name domain.tld www.domain.tld;
server_name ec2...compute.amazonaws.com www.ec2...compute.amazonaws.com
root /home/ubuntu/appname/public;
try_files $uri/index.html $uri #app;
location #app {
proxy_pass http://ec2...us-east-2.compute.amazonaws.com;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $http_host;
proxy_redirect off;
access_log /home/ubuntu/appname/log/nginx.access.log;
error_log /home/ubuntu/appname/log/nginx.error.log;
}
location ~* ^/assets/ {
# Per RFC2616 - 1 year maximum expiry
expires 1y;
add_header Cache-Control public;
# Some browsers still send conditional GET requests if there's a
# Last-Modified header or an ETag header even if they haven't
# reached the expiry date sent in the Expires header.
add_header Last-Modified "";
add_header ETag "";
break;
}
error_page 500 502 503 504 /500.html;
client_max_body_size 4G;
keepalive_timeout 10;
}
I'm pretty new to this so I assume it's something basic, but I've been on this for hours and I'm not sure what to do. Also, if it helps, I'm using puma for my application server.