Error sudo: add-apt-repository: command not found
For Ubuntu 14.04 or later you need to install the software-properties-common package:
sudo apt install software-properties-common
I found the solution in this blog: Ubuntu Server 14.4 (Trusty Tahr) – add-apt-repository: command not found
If you are using an older release of Ubuntu, i.e. before 12.10, it is necessary to install the package python-software-properties.
sudo apt install python-software-properties