Install button in Software Center is disabled

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http://askubuntu.com РFor some unknown reasons Install button is disabled in Software Center and therefore I am unable to install any software from Software Center. But it is possible to install software using command apt-get install package_name. I have Internet connection through 3G modem but I think Software Center doesn't have internet because it shows a notification saying 'No network connection' (Sin conexión de red). In the screenshot below you can see the problem I have described: (HowTos)