Apparently, many users can issue certificates of subdomains of your-server.de. There's a limit of 50 certs per registered domain per week as you can read in the documentation linked in the error message.
Are you the owner/administrator of your-server.de? If so, you can request a rate limit exemption with Let's Encrypt. See the same documentation linked in the error message about that. If not, then you can perhaps request the owner/administrator of your-server.de to do so.
your-server.de is a default domain used by Hetzner as a default placeholder domain, primarily for rDNS purposes. While you can host your website on that placeholder domain name, the general recommendation would be for you to get your own domain name, point that to your server, and then get a certificate for your own domain. This gives you a lot more freedom and flexibility in general, such as being able to use subdomains.
This would also have the side effect of avoiding the rate limit you just hit.
I don't know if Hetzner has a rate limit increase for your-server.de, but based on the error message I guess they don't. I also don't think they will get an override for that domain, considering that this is a placeholder domain name they probably don't actually expect people to host websites on.
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Some challenges have failed.
Ask for help or search for solutions at https://community.letsencrypt.org. See the logfile /var/log/letsencrypt/letsencrypt.log or re-run Certbot with -v for more details.
Please provide the entire output or, if that's everything (in that case the script is very crappy) please provide the contents of the log file mentioned.
In case this wasn't clear, if you don't have a public domain you will either need to buy a public domain from a registrar, or use a service that provides free subdomains.