So you're trying to add a respository in Ubuntu or Linux Mint or similar, and you have a step in the tutorial that you're following that says to run the line add-apt-repository ppa:some/repository and then you sadly get the error message, -bash: add-apt-repository: command not found. And that's frustrating, or it seems. But have no fear and don't worry because this error is easy to get around. All you have to do is what I tell you below.
So just go to your terminal where you saw the error and just run the following command to install the software package:
apt-get install -y software-properties-common
That fixes the problem. So now you can go run the add-apt-repository command and everything will work. Good luck!