apt cannot install koha-common

Start by asking apt what it can actually see: apt-cache policy koha-common. If the candidate is (none) the repository is not configured or not trusted; if there is a candidate but the install fails on unmet dependencies, the repository is fine and your OS release is the mismatch. Those are two different problems with two different fixes.

$ apt-cache policy koha-common koha-common:  Installed: (none)  Candidate: 26.05.00-1  Version table:     26.05.00-1 500        500 https://debian.koha-community.org/koha stable/main amd64 Packages

A candidate and a version table like that means the repository is working. Candidate: (none), or no version table at all, means apt has never successfully read the Koha repository — go to step 2. A candidate that installs and then fails is step 3.

The two halves have to agree: the key file is named inside the repository line, so a key added the old way (apt-key) with a modern line, or the reverse, produces a repository apt refuses to trust.

$ sudo apt -y install wget gnupg ca-certificates apt-transport-https$ wget -qO- https://debian.koha-community.org/koha/gpg.asc \    | sudo gpg --dearmor -o /usr/share/keyrings/koha-keyring.gpg$ echo "deb [signed-by=/usr/share/keyrings/koha-keyring.gpg] \https://debian.koha-community.org/koha stable main" \    | sudo tee /etc/apt/sources.list.d/koha.list$ sudo apt update

The apt update at the end is not optional and is the single most common omission — adding a .list file changes nothing until apt re-reads it. Watch its output: the Koha line should appear without a warning.

koha-common pulls in a long list of Perl modules that come from your distribution, not from Koha. When apt says it "has unmet dependencies" or "requested an impossible situation", it is telling you that this OS release does not carry a version of something Koha needs.

$ sudo apt install koha-common The following packages have unmet dependencies: koha-common : Depends: libmodule-name-perl (>= 2.30) but 2.10-1 is to be installedE: Unable to correct problems, you have held broken packages.

Two things fix this, in this order:

1. 1On Ubuntu, make sure the universe component is enabled. Many Perl modules live there, and a minimal cloud image often has it switched off: sudo add-apt-repository universe && sudo apt update.
2. 2Then try the previous Koha branch. Change stable to oldstable in /etc/apt/sources.list.d/koha.list, run sudo apt update, and install again. The older branch was built against an older set of distribution packages, and on an OS release that is a year or two old it will frequently resolve where stable cannot.

Choosing that release is its own decision — which Ubuntu or Debian version to install Koha on covers how to check before you build the server rather than after.

Koha is not in Debian's or Ubuntu's own archives, so the project builds and hosts its own packages. That is what makes a package install possible in the first place, and it is also the source of every error above: an extra repository is one more thing that has to be configured, signed and matched to your OS.

The suites are branches, not versions. stable follows the current supported Koha release and moves when a new one is cut; oldstable follows the previous one. That is why a server built a year ago and never touched can suddenly fail to update — the suite it is pointed at has moved to a Koha that expects newer distribution packages than the OS underneath it has.

The practical consequence is worth planning for: the operating system release ages faster than Koha does. Building on the newest Ubuntu LTS or Debian stable buys roughly the whole life of that release before this becomes a decision again, and rebuilding a server every few years is a smaller job than fighting a dependency graph.

And if you are looking at this on a Windows machine, none of it applies — see can you install Koha on Windows? first.