Error: Can't open display: (null) when using Xclip to copy ssh public key

Error: Can't open display: (null) when using Xclip to copy ssh public key

I'm following in Generating SSH Keys, it says
sudo apt-get install xclip
# Downloads and installs xclip. If you don't have apt-get, you might need
to use another installer (like yum)
xclip -sel clip < ~/.ssh/id_rsa.pub
# Copies the contents of the id_rsa.pub file to your clipboard
But after I runxclip -sel clip < ~/.ssh/id_rsa.pub I get Error: Can't open
display: (null) What is the problem? I googled around but found nothing
about it