Hit:1 http://archive.canonical.com/ubuntu xenial InRelease
Hit:2 http://archive.ubuntu.com/ubuntu xenial InRelease
Get:3 https://download.01.org/gfx/ubuntu/16.04/main xenial InRelease [3,651 B]
Get:4 http://archive.ubuntu.com/ubuntu xenial-updates InRelease [102 kB]       
Err:3 https://download.01.org/gfx/ubuntu/16.04/main xenial InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 56A3DEF863961D39
Get:5 http://archive.ubuntu.com/ubuntu xenial-backports InRelease [102 kB]
Get:6 http://archive.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Reading package lists... Done     
W: GPG error: https://download.01.org/gfx/ubuntu/16.04/main xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 56A3DEF863961D39
E: The repository 'https://download.01.org/gfx/ubuntu/16.04/main xenial InRelease' is not signed.
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.
red@White:~$