Whonix repos for 15 and 14 down (clearnet & onion):
E: The repository 'https://deb.whonix.org stretch-testers 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.
cc @Patrick