Remote Desktop - Windows to Ubuntu 16.04 LTS
Apparently, the OP's install didn't have all the repos enabled.
In order to enable them and install xrdp, you need to open a terminal (CTRL+ALT+T) and run these commands:
sudo add-apt-repository main
sudo add-apt-repository universe
sudo add-apt-repository restricted
sudo add-apt-repository multiverse
sudo apt-get update
sudo apt-get install xrdp