`koha-create --letsencrypt` checks the system for the `letsencrypt` package in function `check_letsencrypt()`. This piece of code seems to have been added for Debian Jessie. At least from Debian Buster onward (oldoldstable at time of writing) the package `letsencrypt` is a virtual package only and one should go for `certbot` instead. This would become relevant one Koha gets installed on some Ubuntu as this derived distribution does _not_ add a virtual package pointing to `certbot` at least from `focal` onwards (anymore?) hence the check fails.