ussop@Roronoa-Zoro:~$ sudo apt update
[sudo] password for ussop: 
Hit:1 http://archive.canonical.com/ubuntu xenial InRelease
Hit:2 http://archive.canonical.com xenial InRelease                            
Hit:3 http://be.archive.ubuntu.com/ubuntu xenial InRelease                     
Hit:4 http://be.archive.ubuntu.com/ubuntu xenial-updates InRelease             
Hit:5 http://security.ubuntu.com/ubuntu xenial-security InRelease              
Hit:6 http://be.archive.ubuntu.com/ubuntu xenial-backports InRelease     
Get:7 https://repo.skype.com/deb stable InRelease [4.487 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
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.
ussop@Roronoa-Zoro:~$