I had a Ubuntu version 16 running in a virtualbox, and wanted copy text from host to the virtual machine. But it didn’t work. It turns out you need to install the Guest Additions CD image. There is an option from the Devices -> Insert guest additions CD image, but I got then this error message:

Unable to insert the virtual optical disk E:\Program Files\Oracle\VirtualBox\VBoxGuestAdditions.iso into the machine cytomin.

Could not mount the media/drive ‘E:\Program Files\Oracle\VirtualBox/VBoxGuestAdditions.iso’ (VERR_PDM_MEDIA_LOCKED).

The solution was simple enough, just had to run this command:

sudo apt-get install virtualbox-guest-utils