Public key is not available(No_pubkey)

Problem

GPG error: http://ppa.launchpad.net trusty Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 4DF9B28CA252A784 \

Method one

By far the simplest way to handle this now is with Y-PPA-Manager (which now integrates the launchpad-getkeys script with a graphical interface).

To install it, first add the webupd8 repository for this program:
```
sudo add-apt-repository ppa:webupd8team/y-ppa-manager
```

Update your software list and install Y-PPA-Manager:

sudo apt-get update
sudo apt-get install y-ppa-manager

Run y-ppa-manager (i.e. type y-ppa-manager then press enter key).
When the main y-ppa-manager window appears, click on “Advanced.”
From the list of advanced tasks, select “Try to import all missing GPG keys” and click OK.

Method two

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys <PUBKEY>

where <PUBKEY> is your missing public key for repository, e.g. 8BAF9A6F.

Then update

sudo apt-get update