This instance of the problem might be caused by a bug; I recommend considering reporting this as a bug in PolicyKit.
However, this problem can occur with or without a bug, for example, as a consequence of manual PolicyKit configuration changes (or deinstallation). Therefore, it merits an answer here.
This is happening because the PolicyKit service is not running. Here's a couple ways to fix this:
Make sure the necessary PolicyKit packages are installed.
Open a Terminal (Ctrl+Alt+T) and run:
sudo apt-get update && sudo apt-get install policykit-1 policykit-1-gnome
Then reboot and see if the problem is fixed. (Logging out and back in might fix the problem too.)
Run the Software Center with
gksudo instead of PolicyKit.
If that doesn't work, you can still run the Software Center by working around the problem. PolicyKit is one way an authorized user (typically an administrator) can perform actions as
root in a typically configured Ubuntu system, but it is not the only way.
sudo is the other.
For graphical programs, running them with
sudo directly often will work (as in Thomas's answer). But it is not recommended, primarily because it can cause the non-root user's configuration files to become owned by root.
- For graphical programs that don't store configuration files in a user's home directory, this doesn't matter. But most do, and the Software Center is no exception. (It uses
~ is your home directory.)
So it's recommended to use a non-graphical frontend instead:
gksudo, or (if you're running Kubuntu)
So, press Alt+F2 and run:
(This works in all desktop environments, not just Unity, though of course it looks different in others.)
Then you'll be prompted for your password. Enter it, and the Software Center will run as
Please note that this is a little different from the way Software Center usually runs. Ordinarily, it runs as a normal user, and uses PolicyKit to perform actions as root just when it is needed. It would be non-trivial to get it to use
kdesudo to do that.
But running it as
root typically works fine and causes no problems.
Other graphical utilities can be run as
root with graphical
sudo frontends, too.
The problem itself doesn't appear related specifically to the Software Center--any program that uses PolicyKit will fail. Some, like
users-admin, cannot be run successfully as
root. But most can. Here's how to do it, for some of the commonly used utilities:
Software Sources —
Software Updater (called Update Manager before 12.10) —
(If you just want to install updates normally, though, you may be able to do that without becoming root at all, as there is a separate service that allows this.)
System Settings... —