I've followed various guides, e.g.
apt install virtualbox-6.0
It fails with:
root@debian:~# apt-get update Hit:1 http://deb.debian.org/debian buster InRelease Hit:2 http://deb.debian.org/debian-security buster/updates InRelease Hit:3 http://deb.debian.org/debian buster-updates InRelease Hit:4 http://download.virtualbox.org/virtualbox/debian stretch InRelease Get:5 https://download.virtualbox.org/virtualbox/debian buster InRelease [7,733 B] Get:6 https://download.virtualbox.org/virtualbox/debian buster/contrib amd64 Packages [1,521 B] Fetched 9,254 B in 2s (4,145 B/s) Reading package lists... Done root@debian:~# apt-get install virtualbox-6.0 Reading package lists... Done Building dependency tree Reading state information... Done Some packages could not be installed. This may mean that you have requested an impossible situation or if you are using the unstable distribution that some required packages have not yet been created or been moved out of Incoming. The following information may help to resolve the situation: The following packages have unmet dependencies: virtualbox-6.0 : Depends: libcurl3 (>= 7.16.2) but it is not installable Depends: libvpx4 (>= 1.6.0) but it is not installable Recommends: libsdl-ttf2.0-0 but it is not going to be installed Recommends: linux-headers-generic but it is not installable or linux-headers-generic-pae but it is not installable or linux-headers-686-pae but it is not installable or linux-headers-amd64 but it is not going to be installed or linux-headers-2.6-686 but it is not installable or linux-headers-2.6-amd64 but it is not installable or linux-headers but it is not installable Recommends: linux-image but it is not installable E: Unable to correct problems, you have held broken packages.
I've tried to install these dependencies manually, but if I try to install libcurl3, it tells me I can only install libcurl4. If I install libcurl4, it doesn't seem to affect anything, as it requires libcurl3 and not 4.