was not able to manually download and install

 

# does not work
dpkg -i virtualbox-6.0_6.0.10-132072~Debian~stretch_amd64.deb 
# complains about missing libcurl3 (libcurl4 is installed)

but this way it works:

# become fancy root
su - root
# update and upgrade to the latest
apt update
apt upgrade

# install add-apt-repository
apt install software-properties-common

# make the system trust oracle's recent (2016) public key
wget -q https://www.virtualbox.org/download/oracle_vbox_2016.asc -O- | sudo apt-key add -
add-apt-repository "deb http://download.virtualbox.org/virtualbox/debian bionic contrib"

# again
apt update
# install
apt install virtualbox-6.0

cudos to https://tecadmin.net/install-virtualbox-on-debian-10-buster/

admin