Howdy @walterwhite, welcome to the Caddy community.
Your version of Caddy is a few years out of date. The Debian maintainers have made some choices regarding their packaging of Caddy, which includes not opting for the last few years of bugfixes and improvements, but notably they explicitly patch out features like caddy upgrade and caddy add-package which download new versions, circumventing their package management.
If you want to use add-package, you’ll need to install Caddy from a more up-to-date repository (see: Install — Caddy Documentation), or you’ll need to appeal to the Debian package maintainers for support.
As a minor note, I think it also needs the URL to be without scheme, e.g. caddy add-package github.com/porech/caddy-maxmind-geolocation, following the example at Command Line — Caddy Documentation.
Thank you very much. I completely forgot to check if this was the latest version, as I assumed it would be as it is in the Debian package management. After installing version 2.8.2 I was able to add the package successfully.