Can't install aircrack-ng

It has been removed from ubuntu repositories as message says. you can install it by doing following procedure

sudo apt-get install build-essential
sudo apt-get install libssl-dev
wget http://download.aircrack-ng.org/aircrack-ng-1.1.tar.gz
tar -zxvf aircrack-ng-1.1.tar.gz
cd aircrack-ng-1.1

In the aircrack-ng-1.1 directory there is a file called common.mak, use your favorite editor to open the file and scroll down till you see the following line:

CFLAGS ?= -g -W -Wall -Werror -O3

Delete the -Werror variable, so that the line now looks like the following. Save and exit.

CFLAGS ?= -g -W -Wall -O3

Run make and sudo make install in terminal to get aircrack-ng up and running."

For build-essential go to update manager > settings > ubuntu softwares and see if top main is checked.

I have it in a PPA for precise. I did it for another question. If you need directions for adding a PPA they're here.

• Are PPA's safe to add to my system and what are some "red flags" to watch out for?

Looks like it's not in the repositories anymore. I know that aircrack-ng was in the Ubuntu 10.10 repositories, so you might want to try using Ubuntu 10.10 instead of the current version (12.04). But you can also install it in 12.04, using another way. You'll then install aircrack-ng using a .deb file. You just double-click on this file, and it will be opened in the Ubuntu Software Center. To get the .deb file for a 32-bit Ubuntu install (i386), click here. If you're using a 64-bit Ubuntu install (amd64), click here.