mirror_url is not a valid fully-qualified domain name.
Check your repository configuration.
> 404 Not Found [IP: xxx.xxx.xxx.xxx 443]
> Reading package lists... Done
> E: The repository 'https://mirror_url/devuan/merged ceres Release' does
> not have a Release file.
> N: Updating from such a repository can't be done securely, and is
> therefore disabled by default.
> N: See apt-secure(8) manpage for repository creation and user
> configuration details.
> https://pkgmaster.devuan.org/merged/dists/ & mirrors only seem to contain :