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