title: Getting following error after the command sudo apt-get update on Ubuntu 16.04
body: <p>I have no knowledge on Linux commands but I couldn't resist asking why such errors are showing after the command <code>sudo apt-get update</code>:</p>
  
  <blockquote>
    <p>W: The repository '<a href="http://ppa.launchpad.net/bumblebee/stable/ubuntu" rel="noreferrer">http://ppa.launchpad.net/bumblebee/stable/ubuntu</a> xenial Release' does not have a Release file.</p>
    
    <p>N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.</p>
    
    <p>N: See apt-secure(8) manpage for repository creation and user configuration details.</p>
    
    <p>W: The repository '<a href="http://ppa.launchpad.net/ubuntu-x-swat/x-updates/ubuntu" rel="noreferrer">http://ppa.launchpad.net/ubuntu-x-swat/x-updates/ubuntu</a> xenial Release' does not have a Release file.</p>
    
    <p>N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.</p>
    
    <p>N: See apt-secure(8) manpage for repository creation and user configuration details.</p>
    
    <p>W: GPG error: <a href="http://ppa.launchpad.net/umang/indicator-stickynotes/ubuntu" rel="noreferrer">http://ppa.launchpad.net/umang/indicator-stickynotes/ubuntu</a> xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 799A5FD5D1C5997F</p>
    
    <p>W: The repository '<a href="http://ppa.launchpad.net/umang/indicator-stickynotes/ubuntu" rel="noreferrer">http://ppa.launchpad.net/umang/indicator-stickynotes/ubuntu</a> xenial InRelease' is not signed.</p>
    
    <p>N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.</p>
    
    <p>N: See apt-secure(8) manpage for repository creation and user configuration details.</p>
    
    <p>E: Failed to fetch <a href="http://ppa.launchpad.net/bumblebee/stable/ubuntu/dists/xenial/main/binary-amd64/Packages" rel="noreferrer">http://ppa.launchpad.net/bumblebee/stable/ubuntu/dists/xenial/main/binary-amd64/Packages</a>  404  Not Found</p>
    
    <p>E: Failed to fetch <a href="http://ppa.launchpad.net/ubuntu-x-swat/x-updates/ubuntu/dists/xenial/main/binary-amd64/Packages" rel="noreferrer">http://ppa.launchpad.net/ubuntu-x-swat/x-updates/ubuntu/dists/xenial/main/binary-amd64/Packages</a>  404  Not Found</p>
    
    <p>E: Some index files failed to download. They have been ignored, or old ones used instead.</p>
  </blockquote>
  
  <p>I would like to know what's exactly going on with the following errors. An explanation would be very helpful for me.</p>