Add-apt-repository is a python script that adds external repositories to your sources list (/etc/apt/sources.list or /etc/apt/sources.list.d/). If you encounter the command not found error when using this tool, there are two possibilities. Either you made a typo when using the command. Or, the software-properties-common package, which this tool is a part of, is not present on your system. Both cases should be easy to solve. Fix Add-Apt-Repository Command Not Found The software-properties-common package is used to manage software sources. For instance, it provides the add-apt-repository script which lets you easily add and remove PPAs. Install Software-Properties-Common Update your package index and install the latest version of software-properties-common with sudo apt update && sudo apt install software-properties-common If the package was already present, try reinstalling it instead. sudo apt install --reinstall software-properties-common Once that’s done, update your package index again. sudo apt update Test If Add-Apt-Repository Works Finally, verify that the add-apt-repository command works now. We’ll add a PHP PPA to verify this in our case. sudo add-apt-repository ppa:ondrej/php It should be added successfully now. After adding the PPA, don’t forget to update the package index again before installing any packages.