I was trying to use Ubuntu using Oracle Virtualbox 6.1.2.
I cannot make the Ubuntu size full screen. I now have a tiny window with a lot of black background. When I tried inserting guest additions cd image (from Menu -> devices), I got the error message:
Could not mount the media (VERR_PDM_MEDIA_LOCKED)
And when I tried executing the code
sudo apt-get install virtualbox-guest-dkms virtualbox-guest-utils virtualbox-guest-x11 in the Ubuntu terminal, I got the error message,
Some packages could not be installed.
So, how can I increase the present small Ubuntu screen size?