Tried(in the VirtualBox menu)
Device - Connect the disk image to the additional guest OS
and(in the terminal)
sudo apt-get install virtualbox-guest-additions-iso virtualbox-guest-dkms virtualbox-guest-x11 virtualbox-guest-utils
It did not help.
How to fix?
When installed, in the , I had to choose"Start in compatibility mode"because in the normal mode, I could not install it(I tried 2 times on a working PC and home laptop) - the installation did not complete until the end(it gave an error).