This error is Ubuntu telling you its own age. The release you run has been retired from the main servers, and apt is knocking on a door that no longer exists. There is a working stopgap - and an honest conversation about why it is only a stopgap.
One morning sudo apt update starts answering 404 Not Found for Ubuntu's own servers, with the message that the repository "does not have a Release file". You changed nothing - but the calendar did. Your release reached end of life and its files moved house. This page explains how to point your system at the new address, what that does and does not give you, and how to tell this apart from a broken PPA wearing the same error message.
Ubuntu releases do not stay on the main servers forever. When a release reaches end of life - nine months after release for the short-term versions, five years for an LTS - its package files are removed from archive.ubuntu.com and moved to a retirement server, old-releases.ubuntu.com. Your PC is still asking the main address, the main address honestly answers 404 Not Found, and apt reports that the repository "does not have a Release file". Nothing is corrupt. Your Ubuntu is simply older than the main archive keeps.
First confirm your version: run lsb_release -a and note the Codename - that is the word (like impish, kinetic or lunar) apt is failing on in the error.
Understand what you now have. old-releases.ubuntu.com serves the release exactly as it was on its last day - it receives no new security fixes, ever. The stopgap makes apt update succeed and lets you install the software that existed at end of life; it does not make an old Ubuntu safe to keep using on the internet. Treat it as a bridge: it buys you a working machine while you plan the real fix, which is upgrading to a supported LTS release or reinstalling with one. LTS releases are the right choice for machines you do not want to rebuild every nine months.
The wording of this error is identical to the broken-PPA error - same "does not have a Release file" sentence. The difference is the address in the Err line. If the failing address is archive.ubuntu.com or security.ubuntu.com, it is this end-of-life problem. If it is ppa.launchpadcontent.net or any other site, it is a third-party source problem with a different fix. Reading the address before choosing the fix saves you from "repairing" the wrong thing - editing Ubuntu's own entries will never fix a PPA, and removing a PPA will never revive an end-of-life release.
The video shows this 404 on a real machine, the move to old-releases, and the update succeeding again.
▶️ Watch Full VideoRun lsb_release -a to get your release and codename, then check the error: if apt reports a 404 and no Release file for archive.ubuntu.com or security.ubuntu.com with your codename, the release has been retired from the main archive. Short-term Ubuntu releases are supported for nine months and LTS releases for five years, so a version several years old that was never an LTS is almost certainly end of life.
Yes. It is Ubuntu's own retirement archive, run by Canonical, holding the final state of releases that have left the main servers. The packages there are the genuine signed packages the release ended with. What it does not have is anything new: no security fixes are published there after end of life, which is why it is a stopgap rather than a solution.
Usually yes, and the stopgap helps: first point the sources at old-releases so apt works again, install all pending updates for your current release, then use the release upgrade tool to move to the next supported version. Very old releases may need to step through intermediate versions, and at some point a fresh install of a current LTS is honestly less work - back up your files either way before you begin.