Cannot update APT list: "repository no longer has a Release file"

Change testing/updates to testing-security in your sources.list to match http://security-cdn.debian.org/debian-security/dists/testing-security/

Then run apt update instead of apt-get update to interactively accept the various changes.

According to this reddit post this repository name change was introduced in release 10.

If you're upgrading from testing to buster, also be sure to run

apt-get update --allow-releaseinfo-change

If not, apt-get update won't let you update with Buster and will spit out messages like these:

N: Repository 'http://deb.debian.org/debian buster InRelease' changed its 'Version' value from '' to '10.0'

E: Repository 'http://deb.debian.org/debian buster InRelease' changed its 'Suite' value from 'testing' to 'stable'

N: This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.

N: Repository 'http://security.debian.org/debian-security buster/updates InRelease' changed its 'Version' value from '' to '10'

E: Repository 'http://security.debian.org/debian-security buster/updates InRelease' changed its 'Suite' value from 'testing' to 'stable' N: This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.

According to Debian Project News published 1st of July, Debian 10 "Buster" was scheduled to transition from testing to stable on 2019-07-06.

It looks like the transition was in fact performed as scheduled, and so the testing distribution should now be catching up witn unstable, to eventually become Debian 11 "Bullseye". You might be experiencing some temporary side effects because of this transition of the Debian mirrors.

If you want to stay with Debian 10 "Buster", now would be a good time to switch your /etc/apt/sources.list to use the release name buster instead of testing.

Otherwise, you'll soon be getting the raw bleeding-edge stuff from unstable, and you might accidentally get a partial upgrade to proto-"Bullseye".

Also, this is a reminder for anyone using the word stable in their /etc/apt/sources.list to consider whether to change it to stretch and stay with the old version, or read the Release Notes and perform the recommended upgrade steps.