Help! I'm running ubuntu 14.04 LTS With all updates installed. I can't add any sources. When I open software center and go edit>sources>other software. I hit add and enter "deb http://archive.ubuntu.com/ubuntu trusty universe". I get asked for password. I enter it and the source is not showing up in the list. I close the window and go to the software center front page. It tries to update cache but i get an error message saying failed to fetch sources 404 errors.
First reset the repositories by executing the commands below in your Terminal:
sudo echo "deb http://us.archive.ubuntu.com/ubuntu/ trusty main restricted universe multiverse" > /etc/apt/sources.list sudo apt-get update sudo apt-get install -f
Then go back to the Software Center and perform the task you need.