shing@shing-K55A:~$ sudo apt update
[sudo] password for shing: 
Hit:1 http://us.archive.ubuntu.com/ubuntu zesty InRelease
Get:2 http://us.archive.ubuntu.com/ubuntu zesty-updates InRelease [89.2 kB]    
Hit:3 http://ppa.launchpad.net/obsproject/obs-studio/ubuntu zesty InRelease    
Get:4 http://security.ubuntu.com/ubuntu zesty-security InRelease [89.2 kB]     
Hit:5 https://dl.winehq.org/wine-builds/ubuntu zesty InRelease                 
Get:6 https://repo.skype.com/deb stable InRelease [4,486 B]                    
Err:6 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:7 http://us.archive.ubuntu.com/ubuntu zesty-backports InRelease            
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.