wget -qO - https://developer.download.nvidia.com/compute/cuda/repos/ubuntu1604/x86_64/7fa2af80.pub  | sudo apt-key add -
OK
steven@Abby-1:~$ sudo apt-get update
Hit:1 https://nvidia.github.io/libnvidia-container/experimental/ubuntu20.04/amd64  InRelease
Hit:2 https://nvidia.github.io/nvidia-container-runtime/experimental/ubuntu20.04/amd64  InRelease
Get:3 http://security.ubuntu.com/ubuntu focal-security InRelease [107 kB]
Hit:4 http://archive.ubuntu.com/ubuntu focal InRelease
Get:5 http://archive.ubuntu.com/ubuntu focal-updates InRelease [111 kB]
Ign:6 https://developer.download.nvidia.com/compute/cuda/repos/ubuntu20.04/x86_64  InRelease
Err:7 https://developer.download.nvidia.com/compute/cuda/repos/ubuntu20.04/x86_64  Release
  404  Not Found [IP: 152.195.19.142 443]
Get:8 http://archive.ubuntu.com/ubuntu focal-backports InRelease [98.3 kB]
Reading package lists... Done
N: Ignoring file 'Release.gpg' in directory '/etc/apt/sources.list.d/' as it has an invalid filename extension
E: The repository 'https://developer.download.nvidia.com/compute/cuda/repos/ubuntu20.04/x86_64  Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.