Open a terminal and execute:
sudo apt-get install virtualbox
which should work.
Once you have installed it you probably want to install guest additions in the guest OS.
Also you probably want the extension pack which you can get from here:
https://www.virtualbox.org/wiki/Downloads
Make sure you get the correct version.
As of Xenial 16.04, you can also get the extension pack from the Canonical repositories. Go to the terminal and execute:
sudo apt install virtualbox-ext-pack
Therefore, as of Xenial 16.04, you should be able to get virtualbox and the extension pack by typing the following:
sudo apt install virtualbox virtualbox-ext-pack
Following the instructions on your screen.
To install the user interface to manage your virtual boxes use this command
sudo apt install virtualbox-qt
As @bodhi.zazen said in the comments, the tor command is working fine; it's just that Tor is already running. If you read the output from the tor command, you'll see that this is indeed the case.
This is similar to how you can't have to instances of Audacity running at the same time.
If the tor command were really not working, you'd get this output:
user@ubuntu:~$ tor
The program 'tor' is currently not installed. You can install it by typing:
sudo apt-get install tor
Best Answer
Reference: