the error message

basically means: the VM’s screen can not be auto-resized because the VBoxGuestAdditions kernel module drivers software was not properly build & installed.

the solution:

rebuild & reinstall the VBoxGuestAdditions (kernel module) properly:

hostnamectl; # tested on guest (host also being Debian 10)
    Virtualization: oracle
  Operating System: Debian GNU/Linux 10 (buster)
            Kernel: Linux 4.19.0-16-amd64
      Architecture: x86-64
  1. insert the /usr/share/virtualbox/VBoxGuestAdditions.iso
  2. inside the vm:
      • start a terminal:
      • su - root; # become root
      • mkdir /virtualbox; # where to put the files
      • cp -rv /media/cdrom0/* /virtualbox/; # copy the files
  3. update to latest, install build essentials software
    • apt update
    • apt install build-essential module-assistant gcc make perl dkms
    • m-a prepare
    • sh /virtualbox/; # run the compilation & installation
  4. the output that follows should be like
  5. after a
    • shutdown -r now
    • the vm guest resize screen function should work properly