Right, now back to the actual problem at hand: notus-scanner was returning errors on a fresh install of Ubuntu Server 20.04.4 and Kernel 5.4.0-124-generic.
I followed the excellent guide of @libellux at https://libellux.com/openvas/ who provided a superb walkthrough and explanation of the entire process. However, upon starting notus-scanner service, I received errors. Looking back on my notes, I had documented that during the installation process, there were errors during the installation of the tomli module.
Several Python packages, while installed at the start of the build, were not at the correct versions and therefore incompatible with notus-scanner, specifically psutil, python-gnupg, paho-mqtt, deprecated, lxml and redis. One Python package (packaging) was missing completely.
After the initial dependency installations in the guide, I added:
pip install packaging==20.9
python3 -m pip install --upgrade psutil
python3 -m pip install --upgrade python-gnupg
python3 -m pip install --upgrade paho-mqtt
python3 -m pip install --upgrade deprecated
python3 -m pip install --upgrade lxml
python3 -m pip install --upgrade redis
Once i reached the installation of the tomli module, there were no errors, and the build completed without further issue, with notus-scanner starting without any issue.
Important to note that this build was done on Ubuntu 20.04.4 - and while I hope no one else encounters such an error, if you do, I hope this helps you to complete your build.