After installing VirtualBox in windows, and if you use Menu > Devices > Install Guest Additions, most probably you might encounter the below errors:
“Unable to mount the CD/DVD image
Could not mount the media/drive VBoxGuestAdditions.iso (VERR_PDM_MEDIA_LOCKED)”
A simple solution is to open a terminal in Ubuntu, and then type below command:
sudo apt-get install virtualbox-guest-additions-iso
Xinyustudio 
Perfeito! Obrigado!
Thanks a lot!!!!
a possible solution in case of centos7 ?
Fantastic! Thank you!
Debian Stretch doesn’t know the package