I was trying to add the webupd8team/java ppa and my Ubuntu 16.04 system gave me this error:

add-apt-repository: command not found

Well I know there's got to be a way around this error, so I Googled it and this is what I found...

To fix the "add-apt-repository: command not found" error just run the following command:

sudo apt-get install -y software-properties-common

After you do that, the problem should be fixed.

I found this solution at add-apt-repository command not found – Debian & Ubuntu – Quick Fix