Geant4 installation issues

I have installed virtualbox by oracle and within that I have ubuntu 20.04 LTS I am trying to install Geant4 but I keep getting stuck. The website I am referring to is : https://twiki.cern.ch/twiki/bin/view/Main/G4Training
I am unable to run cmake

Any idea what could be going wrong?

you seem to have not installed cmake:
sudo apt-get install cmake