add-apt-repository: command not found

First, switch to the root user and then execute

apt-get install software-properties-common

Then the following prompts will appear:

Reading package lists... Done
Analyzing the dependency tree       
Reading state information... Done       
The following packages will be installed simultaneously:
  libappstream4 packagekit packagekit-tools python3-pycurl python3-software-properties
Suggested packages:
  appstream libcurl4-gnutls-dev python-pycurl-doc python3-pycurl-dbg

Then you can happily execute the add-apt-repository command.