Problem with Debian repository. Missing Release file.
It happened to me before. And seems like it's not working again.
Error:3 http://deb.leap.se/client stretch Release
404 Not Found [...] E: The repository 'http://deb.leap.se/client stretch Release' does no longer 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.