Getting following error after the command sudo apt-get update on Ubuntu 16.04

Question

I have no knowledge on Linux commands but I couldn't resist asking why such errors are showing after the command sudo apt-get update:

W: The repository 'http://ppa.launchpad.net/bumblebee/stable/ubuntu xenial Release' does not have a Release file.

N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.

N: See apt-secure(8) manpage for repository creation and user configuration details.

W: The repository 'http://ppa.launchpad.net/ubuntu-x-swat/x-updates/ubuntu xenial Release' does not have a Release file.

N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.

N: See apt-secure(8) manpage for repository creation and user configuration details.

W: GPG error: http://ppa.launchpad.net/umang/indicator-stickynotes/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 799A5FD5D1C5997F

W: The repository 'http://ppa.launchpad.net/umang/indicator-stickynotes/ubuntu xenial InRelease' is not signed.

N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.

N: See apt-secure(8) manpage for repository creation and user configuration details.

E: Failed to fetch http://ppa.launchpad.net/bumblebee/stable/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found

E: Failed to fetch http://ppa.launchpad.net/ubuntu-x-swat/x-updates/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found

E: Some index files failed to download. They have been ignored, or old ones used instead.

I would like to know what's exactly going on with the following errors. An explanation would be very helpful for me.

Answer 1

I solved this error following theses steps:

Open the Software Updates application; - In "Other Software" tab unmark all options; - In Ubuntu Software tab, "Download from:" choose "Main server" option.

Click in "Close" button and wait a minute until ubuntu reload apt-get.

Done.