If any dependencies is missing, you will see it on the screen. While guest addition is being installed keep focused eye on the output.

Then click on Devices > Insert Guest Additions CD image.

Once all the software updates are completed, open terminal and run following command - sudo apt install gcc make perl.

Click on it and ensure all the pending updates are installed IMPORTANT STEP: If you click on 9 dots (Show Applications), you should see icon for 'Software Update' (spinning circle with A on it).After installation, system will restart but the screen will be small.Opt to do 'normal installation' with following selected - 'Download updates while installing Ubuntu' and 'Install third-party software for graphics and Wi-fi hardware and additional media formats'.Make sure to install VirtualBox Extension pack from. After trying multiple times to have the screen size resolved with Ubuntu 18.04, Virtual Box 6.1 running on Windows 10, here are exact steps that will help: On the internet people are giving different solutions that could end up corrupting the installed guest OS.

Virtualbox extension pack ubuntu 20.04