Add the Debian 11 repository.
echo 'deb http://ftp.de.debian.org/debian bullseye main ' | sudo tee /etc/apt/sources.list.d/debian11.list
sudo apt update
Visit the following URL and download the .deb suitable for Debian,
https://www.virtualbox.org/wiki/Linux_Downloads
Apply the following command to install the VirtualBox Debian package. I assume that the VirtualBox .deb file in the Download folder,
cd Downloads
sudo dpkg -i virtualbox*
Find the VirtualBox in the application launcher.