OpenCV installation in ubuntu 18.04 on Jetson Nano

Question

when I am running this ubuntu command below error is coming:

For installation I am using this link: https://jkjung-avt.github.io/opencv-on-nano/

Err:12 http://old-releases.ubuntu.com/ubuntu xenial-security Release                                                                                                                                       
  404  Not Found [IP: 91.189.88.248 80]
Reading package lists... Done                                                                                                                                                                              
E: The repository 'http://old-releases.ubuntu.com/ubuntu xenial-security Release' does not have a Release file.