My domain is:happyfm.eu
I ran this command: Encrypt website via Plesk
It produced this output: SSL/TLS-Zertifikat konnte für happyfm.eu nicht ausgestellt werden. Details: Could not assign certificate to the domain. stdout: , stderr: Certificate with name Lets Encrypt happyfm.eu is not found
My hosting provider, if applicable, is: https://living-bots.net
I can login to a root shell on my machine: i don't think so
I'm using a control panel to manage my site: Plesk
Sincerely yours,
Lucas
This probably is an issue you should take up with your hosting provider. If you're on a shared hosting without root access, there probably isn't anything you could do anyway unfortunately.
Also, a search for that error message + "Plesk" didn't find anything with Google. Which strengthens my thought that the best place to ask about this error is at your hosting provider.
Also, you might want to ask your hosting provider what they are doing with their Let's Encrypt client. If you look at the recently issued certificates for your domain:
you can see you've got a lot of certificates issued recently. At least 9 (!!!) certificates since 30 Oktober. All which are not currently in use.
The certificates were issued by me because they didn't work. My hoster told me that there is nothing he can do about it and that the current certificate has to expire first, then I can make a new one. He also said that if I have a question about it, I should ask here.
When can I request a new certificate so that it works again and can I somehow delete the certificates that are currently not in use?
Hi @Lqxa
that's wrong. Your hoster doesn't know some basics.
Your hoster has to fix it.
Certificate creation works (see the list @Osiris has shared) , certificate installation not. But installation is a local problem. Shared hosting -> you can't do that if your Plesk is buggy.
So your Plesk is buggy -> it's the job of your hoster to fix that.
I'll get back to my hoster, thank you very much for the help.
In some clients like Certbot, the default behavior is not to request a new certificate unless the old certificate is less than 30 days away from expiring. Of course, that's different from already expired! And this behavior is a default in the client software and can be overriden (for example, in Certbot with --force-renew).
So it's definitely true that there is software that's reluctant to issue a new certificate when the old certificate still has a lot of validity left, but this should never be an absolute limit. It's not a Let's Encrypt CA policy!
We are more than happy to help how we can. 
Sorry we couldn't immediately do anything for you. If your hoster has questions (or needs help, which it appears they do), they are also welcome to come ask us what they may. We might be able to improve the experience of all of their users.