did you set up letsencrypt/certbot in the first place to write files to /usr/local/etc/letsencrypt/live/domain.org/cert.pem? If so, did you take care to replace domain.org by the actual domain you are using?

The documentation you linked looks a bit funny in that the first command writes to private key/cert to privkey.pem and cert.pem, but then the second command tries to read in a (likely) certbot-created certificate. I guess if you followed the steps you need to replace usr/local/etc/letsencrypt/live/domain.org/cert.pem in the second command by the cert.pem created in the first one?

I'd recommend using a reverse proxy even if you just have 1 service. The swag container from Linuxserver is good, nginx proxy manager is probably the easiest, both automate the cert and renewal