Please follow the below template, it will help us to help you!
Expected Behaviour:
Raspberry PI 3B+, as always log into the console and run pihole -up
Actual Behaviour:
Pi-hole and web interface update regularly but when trying to update FTL from 5.8.1 to 5.9 this error message pops up
[✗] Update local cache of available packages
Error: Unable to update package cache. Please try "sudo apt-get update"
Unable to complete update, please contact Pi-hole Support
Upon trying to run sudo apt-get update this message is shown:
Reading package lists... Done
E: Repository 'http://raspbian.raspberrypi.org/raspbian buster InRelease' changed its 'Suite' value from 'stable' to 'oldstable'
N: This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.
If you look for information how to upgrade your OS including or how to switch to a new release, you'd have to refer to your respective OS's documentation and support.
Since you seem to be using Raspberry Pi OS, note that the latest Raspberry Pi OS image as available from raspberrypi.org is still dating from May 7th 2021, so Buster still. Also, note that he RPi Foundation discourages in-place upgrades to new releases. That probably means starting with a fresh image of Raspberry Pi OS 11/Bullseye once that is released.