Question: I've completed a fresh installation of Ubuntu 16.04. After system installation, I've installed some other necessary applications for me (including i.e Opera browser, Intel graphics driver etc.). Now when I'm trying to update the system from terminal using `sudo apt-get update`. It gives me the following error 

```
W: The repository 'https://deb.opera.com/opera-stable stable 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: https://download.01.org/gfx/ubuntu/16.04/main xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 56A3DEF863961D39
W: The repository 'https://download.01.org/gfx/ubuntu/16.04/main 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 https://deb.opera.com/opera-stable/dists/stable/non-free/binary-amd64/Packages  404  Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.

```

How can I overcome this issue?

Answer: 
You have errors in both Opera's and Intel's PPAs.

1. `wget -qO- https://deb.opera.com/archive.key | sudo apt-key add -`
[Source](https://deb.opera.com/manual.html)
2. Intel Graphics drivers are already installed and are open-source, updated along the rest of the system whenever deemed necessary. This are drivers, not applications. You're certainly referring to the Intel Graphics for Linux tool you already used and certainly don't need, and not an app. Ubuntu 16.04 comes with fairly recent Intel drivers. There's little, if anything, to gain from the minor revision updates provided at Intel's PPA.

If you want to use it anyway then please follow **all** the instructions at the download's page. Here's what you should do now, in a nutshell,

* `wget --no-check-certificate https://download.01.org/gfx/RPM-GPG-KEY-ilg-4 -O - | sudo apt-key add -`

	3. `sudo apt update && sudo apt full-upgrade`

No need to run the installer again. FYI, all it does is opens a GUI (nothing wrong about that), adds the PPA, update && dist-upgrade. This is where it fails when users don't follow the instructions adding the keys as above before running the installer.
Your path to the opera repository seems to be wrong - you get a 404 when you try it in a browser.

The correct definition seems to be 

```
deb https://deb.opera.com/opera-stable/opera/dists/stable non-free

```

although I could not test.