The error -bash: add-apt-repository: command not found is very common on Ubuntu 18.04 and Ubuntu 16.04. The error is occurring on minimal versions of the above mentioned operating systems. This is due to a mission dependency. The error doesn't seems to be occurring on any of my recommended VPS providers as the dependency is pre-installed on even on the minimal OS server.

However can simply fix the error by installing software-properties-common package. Following command should do it,

apt install software-properties-common

Update apt cache,

apt update

Now you can the repository you were trying to add. I came across this error while configuring a server for a client. I was hired to install CPVLab Pro with Virtualmin and Nginx on Ubuntu 18.04.