Step 1. Install required packages.
Before installing VirtualBox Guest Additions on Ubuntu 18.04 we need to install some required packages. So open the terminal and run the following command.
sudo apt install linux-headers-$(uname -r) build-essential dkms
Step 2. Install Guest Additions using virtual box.
In order To install VirtualBox Guest Additions on Ubuntu virtual machine, Got to VirtualBox - select the Devices from VirtualBox host application menu - click Insert Guest Additions CD image.
Step 3. This will open a pop up window and it will ask you to run it. Press Run button (see the picture below)
After pressing Run button it will install VirtualBox Guest Additions on Ubuntu 18.04. Press enter to close the window.
Step 4. Shut down your Ubuntu 18.04 Guest.
Step 5. Start Ubuntu 18.04 virtual machine and check if copy and paste works.
Step 3 has no popup window