Please modify /etc/apt/sources.list
As to pcDuino8 Uno, add the following line:
deb http://ubuntu.pcduino.com/pcduino8/ pcduino main
As to pcDuino3/2/1, add the following lines:
deb http://ubuntu.pcduino.com/pcduino3/ pcduino main
deb-src http://ubuntu.pcduino.com/pcduino3/ pcduino main
After that, update system:
sudo apt-get update