Thsi question hs been sked hundreds of times. Before posting a new question it is a good idea to see if ayone else has been having the same problem and what has been answered for it.
From a post asking this question just before you asked:
Its an error at Apples end nothing wrong with your system. Usually shows up when Apple pushes a lot of updates at once. Wait a while and try again but it may take overnight before it clears up.
If your desperate for the updates and its not an App Store purchase you can usually find the dmg file of the update on the Apple support site. Search around.