# How can i fix this error when i run sudo apt-get update command? [duplicate]

W: Failed to fetch http://ppa.launchpad.net/docky-core/stable/ubuntu/dists/raring/main/source/Sources  404  Not Found

If you follow these links, you can find out they are indeed missing. There is, however, a repo for precise. So I would suggest looking into /etc/apt/sources.list (Alt+F2 - gksu gedit /etc/apt/sources.list) for this repository line and changing there raring to precise. Then sudo apt-get update one more time to see if this made desired effect.