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!
Downloading the older version of Virtual Box worked for me (Version 5) https://www.virtualbox.org/wiki/Download_Old_Builds_5_2
If the guest vm is not showing any installer popup while you click install on guest addition folder or while running from the terminal as,
sudo ./runasroot.sh or
Try this command for the file in that folder with VBoxLinuxAdditions.run as,
sudo ./VBoxLinuxAdditions.run install
Hope this works with all the installations. After the installations finishes just restart and you will be shown all the features of the Guest Additions.