apt "E: Unable to locate package": update, enable the component, or find its real name

1. Update the list

This solves most cases, particularly on a fresh server or container, where the list is empty:

sudo apt update

Read its output, since an error here is the actual problem:

2. Is the name right?

Names on Debian and Ubuntu often differ from what you’d expect, and they’re case-sensitive:

apt search '^nginx'
apt-cache search postgresql | grep client
apt list 'python3-req*'

Some common differences:

Know the command, and not the package? apt-file finds it:

sudo apt install apt-file
sudo apt-file update
apt-file search bin/dig

Don’t use a version number if the package doesn’t have it. python3.11 exists only on some releases.

3. Enable the component (Ubuntu and Debian)

Repositories are divided into parts, and not all are on by default.

On Ubuntu, universe holds many community packages. Minimal images and cloud images may have it off:

sudo add-apt-repository universe
sudo apt update

add-apt-repository comes from software-properties-common, if it isn’t installed.

On Debian, non-free drivers and tools live in contrib, non-free and non-free-firmware. Add them to the lines in /etc/apt/sources.list:

deb http://deb.debian.org/debian bookworm main contrib non-free non-free-firmware

See what’s currently enabled:

grep -rh '^deb\|^Types\|^Components' /etc/apt/sources.list /etc/apt/sources.list.d/

Newer systems also use .sources files in that folder, with a Components: line.

4. It isn’t in your release

Packages come and go between releases. Check which release you have, and what it offers:

lsb_release -a
apt policy some-package

Look it up on packages.ubuntu.com or packages.debian.org, which also show which release has which version.

Many tools aren’t in the distribution at all, and come from the vendor’s own repository, such as Docker, Terraform, Kubernetes, or a current PostgreSQL. Follow their instructions, which add a key and a source file. A wrong key gives NO_PUBKEY.

A release that’s out of support has moved to another server, so apt update fails with does not have a Release file or 404s. For Ubuntu, it moved to old-releases.ubuntu.com, and for Debian to archive.debian.org. The real fix is upgrading, because nothing there gets security updates.

A PPA on the wrong release gives the same message. Remove it with sudo add-apt-repository --remove ppa:name/here.

5. Architecture

A package for amd64 isn’t found on arm64, such as on a Raspberry Pi or an AWS Graviton instance. Check it:

dpkg --print-architecture
apt-cache policy some-package

A vendor’s source line can be limited, as in deb [arch=amd64] ..., which hides it from other architectures.

6. In a Dockerfile

Official images ship without the list, to stay small. So it’s always empty at the start, and you need the update in the same RUN:

RUN apt-get update && apt-get install -y --no-install-recommends curl \
    && rm -rf /var/lib/apt/lists/*

Keeping update on a separate line gets it cached, and a later build then uses an old list, so packages can’t be found or downloaded. apt also warns about being used in scripts, so use apt-get here.

Your base image might not be Debian-based at all. If apt-get itself isn’t found, you’re probably on Alpine (apk add) or RHEL (dnf install).