pip3 has no installation candidate
// Run
sudo nano /etc/apt/sources.list
// And add these lines.
deb http://archive.ubuntu.com/ubuntu bionic main universe
deb http://archive.ubuntu.com/ubuntu bionic-security main universe
deb http://archive.ubuntu.com/ubuntu bionic-updates main universe
// Press Ctrl+O to save the file. Press Ctrl+X to quit nano. Then run:
sudo apt update
sudo apt install python3-pip