Unsupported OS detected: Raspian 9

[hgood]

The issue I am facing:
When I run pihole -up to upgrade to the latest version the installation fails withe the message
"Unsupported OS detected: Raspian 9"

Details about my system:
Raspian 9 on a PI 2

What I have changed since installing Pi-hole:
Regularly update of the underlaying Raspian OS and pihole

[Bucking_Horn]

As mentioned in the release notes and announced well before the release, Raspbian 9 is not a supported OS anymore.

Long-Term-Support for Debian 9/Stretch was terminated by Debian on 2022-06-30.

See also our related announcements:

Bye Bye Debian Stretch :: Looking for testers

Bye Bye Debian Stretch :: Looking for testers We're going to upgrade Pi-hole's CI workflows used for generating the pre-compiled FTL binaries with the next release. We do this to install important upgrades in the build environment. We update not only libraries but also the entire compiler suite. By this update, the minimum required operating system is changed from Debian Stretch (9) to Buster (10). Debian 9 was initially released on 2017-06-17. It was superseded by Debian Buster on 2019-07-06…

Upcoming Pi-hole changes

Originally published at: Upcoming Pi-hole changes – Pi-hole We are fast approaching another release (soon™), and we’ve been very busy since the last one. There are a number of smaller changes, but we thought it would be nice to share some of the bigger/more important changes ahead of time, so that they are not missed in amongst the release notes. Saying Goodbye to Debian Stretch We already posted about this on discourse recently, however it is worth repeating that the next release of Pi-h…

I'd recommend to use Pi-hole's Teleporter to backup and download your Pi-hole configuration to a safe location.
Then download the latest Raspberry Pi OS Lite image (currently 11/Bullseye) and write it to your SD card, install Pi-hole and import your Pi-hole configuration backup.