I’m not sure why the authors of that page decided to place executable scripts under /etc, but that’s enough to make me not want to use that guide.
Why do you have a file named office.acme.ch.sh? The name is supposed to include your DNS domain name. I find it extremely unlikely that you are the registrant of the acme.ch domain. DNS confirms that there is no such hostname as office.acme.ch.
You can learn about the webroot in the Certbot documentation. User Guide — Certbot 3.1.0 documentation
You may want to visit the Let’s Encrypt Community. They will be better equipped to help you obtain certificates from their CA.