This post describes how you can install Virtualbox Guest Additions on Ubuntu 20.04.
Installing Virtualbox Guest Additions on Ubuntu 20.04
To install Virtualbox Guest Additions on Ubuntu 20.04 you will want to enable the multiverse repository.
You can do this by running the following command.
sudo add-apt-repository multiverse
Once this is done you will then be able to install the DKMS module and the guest utilities.
sudo apt install virtualbox-guest-dkms virtualbox-guest-x11 virtualbox-guest-utils
Once done and after rebooting you should have access to the full suite of guest additions.