Using latest Virtual Box 6.1 on Windows10 host, using Ubuntu 18.04 guest
First I had installed by downloading How To Install VirtualBox Guest Additions on Ubuntu 18.04
sudo apt install build-essential dkms linux-headers-$(uname -r)
sudo sh ./VBoxLinuxAdditions.run --nox11
The install was successful, It should have worked but didn't
Next I did (as suggested above):
sudo apt-get install virtualbox-guest-dkms
sudo apt-get install virtualbox-guest-dkms virtualbox-guest-utils virtualbox-guest-x11
This also did not work, I have rebooted and all that, enabled bidirectional - clipboard, drag-n-drop, etc.
Not only cut-n-paste does not work, I cant even scroll my mouse pointer above the prompt - so essentially makes the VM useless.
You would expect something like this to work by default, but unfortunately not, even after all the hoops.