duplicate upstream "backend" in /etc/nginx/sites-enabled installing Mastodon - nginx

Question

I have already installed Mastodon, but I'm in the step of setting up nginx according to this doc https://github.com/mastodon/documentation/blob/master/content/en/admin/install.md
I already changed the mastodon file to put my domain and uncomment the lines of the certificate.
so it looks like this:
# Uncomment these lines once you acquire a certificate:
ssl_certificate /etc/letsencrypt/live/my.domain/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/my.domain/privkey.pem;
In the upper part of the file I have this:
map $http_upgrade $connection_upgrade {
default upgrade;
'' close;
}
upstream backend {
server 127.0.0.1:3000 fail_timeout=0;
}
upstream streaming {
server 127.0.0.1:4000 fail_timeout=0;
}
In /etc/nginx/sites-enabled/ there are three files: default which have I don't know what; and mastodon and my.domain.conf that looks exactly the same, as described above.
Once I modified the mastodon file to put my.domain when it appears, I was told to restart nginx so I run:
sudo systemctl restart nginx
I got this:
Job for nginx.service failed because the control process exited with error code.
See "systemctl status nginx.service" and "journalctl -xeu nginx.service" for details.
so I run "systemctl status nginx.service" and got this:
× nginx.service - A high performance web server and a reverse proxy server
Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled)
Active: failed (Result: exit-code) since Sat 2022-11-12 22:28:59 UTC; 15s ago
Docs: man:nginx(8)
Process: 144802 ExecStartPre=/usr/sbin/nginx -t -q -g daemon on; master_process on; (code=exited, status=1/FAILURE)
CPU: 40ms
Nov 12 22:28:59 my-instance-1 systemd[1]: Starting A high performance web server and a reverse proxy server...
Nov 12 22:28:59 my-instance-1 nginx[144802]: nginx: [emerg] duplicate upstream "backend" in /etc/nginx/sites-enabled/my.domain.conf:6
Nov 12 22:28:59 my-instance-1 nginx[144802]: nginx: configuration file /etc/nginx/nginx.conf test failed
Nov 12 22:28:59 my-instance-1 systemd[1]: nginx.service: Control process exited, code=exited, status=1/FAILURE
Nov 12 22:28:59 my-instance-1 systemd[1]: nginx.service: Failed with result 'exit-code'.
Nov 12 22:28:59 my-instance-1 systemd[1]: Failed to start A high performance web server and a reverse proxy server.
I don't know why it says I have duplicated upstream backend if it appears only once in the file. I need to be able to restart my nginx.
UPDATE
Current state is that in the picture. I changed in the mastodon file in sites-AVAILABLE all example.com by my.domain (lines 28, 37,38 aprox). cert lines are uncommented and nginx is throwing this error.
My current instance looks like this, I created a user but the mail verification link got ERR_CERT_COMMON_NAME_INVALID.