I am having really annoying problem and have spent hours trying to fix. I have searched ad nauseum and found others with same problem but none of the solutions work. I go to Devices to Install Guest Additions and find that instead it says Insert Guest Additions CD. When I try that it says it cannot insert and asks if I want to force mount. I try that and I get error that it cannot force unmount because it is locked (I get an error that is similar to this). What the heck is going on? Appreciate any advice. Thank you!
An alternate method to install the Guest additions is to use the Ubuntu repository versions, log into the VM to install the following packages:
sudo apt-get install virtualbox-guest-dkms virtualbox-guest-utils virtualbox-guest-x11
Finally restart the VM to complete the installation.
Downloading the older version of Virtual Box worked for me (Version 5) https://www.virtualbox.org/wiki/Download_Old_Builds_5_2