Whonix 15 VIrtualBox builds currently broken since VirtualBox was removed from Debian buster. Will probably be re-added in buster-backports. Workaround until fix  gets developed:
Add virtualbox packages to
whonix_build_script_skip_package_install Whonix build script variable or by manually continuing on VirtualBox guest additions package installation.
 The fix will be: Re-add the source code similar like in Whonix 14 build process: download from wherever available (buster-backports in future, sid for now and add to Whonix local (and remote) repository.