There's no way for letsencrypt to issue you a certificate because they can't connect to it from outside.
We gave up and paid for a certificate.
Other automated systems like ACM[3] are very easy, but assume you've bought into a more expensive infrastructure, and often only support their (AWS's in this case) services.
And at the other extreme, there are a plethora of simple web hosting packages[4] that only serve very cookie cutter setups.
That leaves a decent number of smaller organizations or individuals that aren't really able to take advantage of automation because it doesn't support their configuration, or because they don't have the expertise to set it up correctly. So they are often stuck renewing manually.
[1]: https://letsencrypt.org/docs/ [2]: https://letsencrypt.org/getting-started/ [3]: https://aws.amazon.com/certificate-manager/ [4]: https://www.gandi.net/en/simple-hosting
And every time someone says, can't everyone just use LetsEncrypt, they mean, why haven't they automated it?
* https://en.wikipedia.org/wiki/Enrollment_over_Secure_Transpo...
* https://en.wikipedia.org/wiki/Simple_Certificate_Enrollment_...
ACME is the third kick at the can AFAICT. Some people could perhaps be excused for thinking ACME/LE was just another flash in the pan.
Up until recently, F5's BIG-IP LTM didn't have an automated way to do it either, but since it's an appliance that is basically running on top of CentOS, you can SSH in and get a bash shell and run something like dehydrated (minimal dependencies):
* https://lnxgeek.org/doku.php/indexes:let_s_encrypt_-_how_to_...
There's stuff out there besides just Apache and Nginx on a Linux system, and it can't all be automated.
[0]: https://www.troyhunt.com/extended-validation-certificates-ar...