I believe the real problem is not exactly that software-properties-common is not installed, but that it doesn't depend on python3-software-properties. This way, even if the add-apt-repository binary is installed, softwareproperties.SoftwareProperties python module might not be available since it's provided by python3-software-properties package.
I believe the real problem is not exactly that software- properties- common is not installed, but that it doesn't depend on python3- software- properties. This way, even if the add-apt-repository binary is installed, softwarepropert ies.SoftwarePro perties python module might not be available since it's provided by python3- software- properties package.