

The main idea on a device running something like Graphene OS is that you are already in a state of using minimal, if not at all using Google Cloud services, including data backups. It’s intended in tandem with modifications like GMS, GPS (if optionally installed into a given user, work profile) running as an unprivileged, permission-based application. If someone is taking their data privacy and security seriously enough to consider using a duress PIN and flashed their phone with something along the lines of Graphene OS, would they be likely to have heavy reliance to Google’s Cloud offerings?
You’re likely looking for this docs section for Caddy. The failure is the automated request to populate Caddy’s root CA cert to the host system, but obviously failed as it doesn’t have root permissions. As the docs state, if you intend to use the local HTTPS functionality of Caddy, you can manually run
caddy trust
privileged in order to populate the Caddy root CA cert manually. If you intend to disable the local HTTPS functionality (such as if you’re running Caddy behind a http reverse proxy), you can ignore the mail message.