In ubuntu system, sometimes you might need to add a repository in system source list and do some installation.
But when you wanna try to use add-apt-repository, your system just said that it can’t find this command.
In this situation, you need to use the following command to solve.
$ sudo apt-get install python-software-properties
Let your system have python properties. And then,
$ sudo apt-get install software-properties-common
Then you can happily use add-apt-repository.