That is not what the instructions say, those are not links nor commands to run on the terminal, you have to add one of those lines (which one to choose depends on your specific Ubuntu version) to your /etc/apt/sources.list file and then run these commands on the terminal.
sudo apt-get update
sudo apt-get install r-base
sudo apt-get install r-base-dev
R is fully supported on the LTS versions only but I think you can use the Bionic repositories on Ubuntu 19