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I had the exact same problem and solved it by installing The Four Main Repositories:

1. Main - Canonical-supported free and open-source software.
2. Universe - Community-maintained free and open-source software.
3. Restricted - Proprietary drivers for devices.
4. Multiverse - Software restricted by copyright or legal issues.

To install these repositories one method is to go through the "Ubuntu Software Center". Open the software center, then from the Edit menu select "Software Sources" and select checkboxes next to the repository names. Complete Instruction can be found here: https://help.ubuntu.com/community/Repositories/Ubuntu

However, when I controlled the "Ubuntu Software Center" these repositories were already checked! When I tried to close the window it throw an error that cannot update. I solved that problem by changing the Download From to Main Server.