I just don't know what is happening but I can't download Xcode.
I did click on the update button on the App Store, but it did not do nothing.
What should I do?
Maybe it is just temporary bug… wait sometime, and try again. Or try to restart AppStore application.
But, if you need it right now, and nothing helps - you can download dmg file with latest XCode from: https://developer.apple.com/downloads