Ok, I completely deleted the /ardupilot directory from my system.
Then per Ardupilot instructions: created a fork of the main Ardupilot repository and my GitHub account and cloned the fork.
The Tools/environment_install/install-prereqs-ubuntu.sh -y script works now ye!
A question remains: Is the latest stable version of ArduSub included in the main Ardupilot repository or should I follow the instructions from Ardusub:
git fetch --tags
git checkout ArduSub-stable -b new-branch
git submodule update --init --recursive
Advice please!
Zac