Welcome to TurnKey. I hope that you're not finding the learning curve too steep and it's giving you some value.

Ok, so first up perhaps it's useful for some background and context?! TurnKey is essentially a customised system made from a few different sources, plus a few bits of our own software.

The base OS is Debian. The latest v15.0 release (i.e recently announced here and here) is based on Debian 9/Stretch and the prior v14.2 release was based on Debian 8/Jessie. So under the hood, TurnKey is basically a customised minimal Debian OS intended for use as a headless (i.e. no directly attached monitor/keyboard/etc) server.

With that out of the way, TBH, I'm not totally sure what you mean re installing caldav. I'm assuming from what you've written that you're using Webmin to install packages from the Debian repos? And you found one called "caldav" and have installed it. Please correct me if I'm wrong.

Assuming I'm on the right track, I've just had a quick look at the Debian repos and I could not find a package which is explicitly called "caldav". The most likely looking one to my eye is perhaps python-caldav? If I'm still on the right track, then I'm pretty sure I know why you can't see any way to interact with it via Webmin. That's because it's a python (programming language) library for interacting with a caldav server. Probably of no value at all to you, unless you've found a program (written in python) which you want to use to pull caldav info from a remote server! Or yo're going to write your own...

Looking at the other packages with caldav in the name, one that does appear to be a caldav server (sort of - so perhaps is the one you mean?) is cyrus-caldav. That appears to add caldav functionality to the Cyrus mailsystem (and IMAP email server with support for CalDav). However, I'm am not aware of there being a Webmin module for managing that and it doesn't appear to have any sort of Web UI. Having said that, a quick look on the Webmin website suggested that there are a couple of third party Webmin modules. Although one was last updated 10 years ago, and the other is even older! So I'm not sure how useful (or secure) they'll be?! Otherwise, your next best administration option will be installing cyrus-admin (a commandline admin tool). So not really sure if any of that is of value to you?!

Perhaps I've missed the mark completely?! If so, please set me straight. If you have other questions, please ask and I'll do my best to answer. :)