While trying to install Oracle Java on an Ubuntu system, I used the command:

add-apt-repository ppa:webupd8team/java

And Ubuntu responded with:

add-apt-repository: command not found

Well I've never seen that before. Usually add-apt-repository goes off without a hitch. 

But apparently you have to install something else (sometimes) before you can use the add-apt-repository command.

So I did what Franc Drobnič said to do in this thread and I ran the following command:

apt-get install software-properties-common

Well then that fixed it and I was then able to run the add-apt-repository command.