After inserting the virtual disk to install additions, you do:
And you get this error:
Kernel headers not found for target kernel 4.19.0-9-amd64. Please install them and execute /sbin/rcvboxadd setup
Install the following packages:
sudo apt-get install build-essential linux-headers-`uname -r`
After this, reboot the computer and try to install again. Now it should install without errors.
Remember to reboot one last time to enable the VBox extensions and you should see that dynamic resizing is now activated