sudo apt update
[sudo] password for immu: 
Hit:1 http://mirrors.nayatel.com/ubuntu zesty InRelease
Hit:2 http://mirrors.nayatel.com/ubuntu zesty-updates InRelease                
Hit:3 http://mirrors.nayatel.com/ubuntu zesty-backports InRelease              
Ign:4 http://dl.google.com/linux/chrome/deb stable InRelease                   
Hit:5 http://archive.canonical.com/ubuntu zesty InRelease                
Hit:6 http://mirrors.nayatel.com/ubuntu zesty-security InRelease         
Get:7 https://repo.skype.com/deb stable InRelease [4,486 B]              
Err:7 https://repo.skype.com/deb stable InRelease          
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 1F3045A5DF7587C3
Hit:8 http://dl.google.com/linux/chrome/deb stable Release
Reading package lists... Done
W: GPG error: https://repo.skype.com/deb stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 1F3045A5DF7587C3
E: The repository 'https://repo.skype.com/deb stable 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.