Unable to install Vulkan

There are repositories for 2 LTS versions and these 2 lines are mandatory before apt install works

• To add the key for the repository:

  wget -qO - http://packages.lunarg.com/lunarg-signing-key-pub.asc | sudo apt-key add -
  

• either Ubuntu 18.04 (Bionic Beaver):

  sudo wget -qO /etc/apt/sources.list.d/lunarg-vulkan-bionic.list http://packages.lunarg.com/vulkan/lunarg-vulkan-bionic.list
  

• or Ubuntu 20.04 (Focal Fossa):

  sudo wget -qO /etc/apt/sources.list.d/lunarg-vulkan-focal.list http://packages.lunarg.com/vulkan/lunarg-vulkan-focal.list
  

and then ...

sudo apt update
sudo apt install vulkan-sdk