SSL certificate generation on an EC2 machine with NGINX

Question

I am trying to generate an ssl certificate on an AWS EC2 nano machine configured with a NGINX server. My NGINX configuration file is as follows:
server {
listen 80 default_server;
listen [::]:80 default_server;
root /var/www/html;
server_name my_server_name.com;
}
When I check my Nginx configuration with the following command :
nginx -t
this error is returned:
nginx: [emerg] a duplicate default server for 0.0.0.0:80 in /etc/nginx/sites-enabled/default:22 nginx: configuration file /etc/nginx/nginx.conf test failed

Of course, when I try to go further in generating the SSL certificate with the following command:
certbot certonly --dry-run
the following error is returned:
Error while running nginx -c /etc/nginx/nginx.conf -t.

What changes/commands do I need to make to generate the SSL certificate ?

I have modified my Nginx configuration file in a thousand ways, without success.