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/ bionic main restricted
Then the URL that apt will use to get the Releases file will be:
http://localhost:8081/repository/ubuntu-archives/dists/bionic/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/bionic/Release