VirtualBox 6.1.26 from virtualbox.org Debian buster repository cannot be installed on Debian bullseye repository due to dependency issues.
Will try and probably use Debian fasttrack repository for Whonix 16 which will be Debian bullseye based.
(Installing VirtualBox on Debian 10 / Debian buster / buster-backports / fasttrack)
(Install Debian fasttrack-archive-keyring / enable Debian fasttrack repository by default?)