Error "repository does not have a Release file" using apt proxy repository

Using the apt package management tool with a Nexus 3 Apt format proxy repository may return an error similar to the following:

E: The repository 'http://localhost:8081/repository/alpine-proxy buster Release' does not have a Release file.

A Debian repository will contain a Release file for each component. If the remote repository does not contain Release files then the Apt client will return these errors. Debian repository anatomy is described here:

https://wiki.debian.org/DebianRepository

If the repository contains latest-releases.yaml files then it most likely supports APK rather than Apt.

Debugging tip...

If you have this line in sources.list:

deb http://localhost:8081/repository/ubuntu-archives/ groovy-updates main restricted

Then the URL that apt will use to get the Releases file will be:

http://localhost:8081/repository/ubuntu-archives/dists/groovy-updates/Release 

You can take the path of that file and append it to to remote URL configured in the proxy to test that the file exists on the remote:

http://us.archive.ubuntu.com/ubuntu/dists/groovy-updates/Release