Start the virtual machine and make sure that the Guest Additions are inserted. In Virtualbox 6.1.18 this can be done with “Devices” > “Insert Guest Additions” via the top menu. After that insert the following commands into a Terminal:

user@virtualbox:~$ sudo apt install gcc make perl dkms
user@virtualbox:~$ sudo '/media/user/VBox_GAs_6.1.18/VBoxLinuxAdditions.run' 
user@virtualbox:~$ sudo reboot

And that’s it! You should now be able to work with your virtual machine using features like full screen and many others.