I cannot download Xcode from the App Store

Maybe it is just temporary bug… wait some time, and try again. Or try to restart the Mac App Store application.

But, if you need it right now, and nothing helps - you can download a dmg file with latest Xcode from: https://developer.apple.com/downloads

The problem is that there is an issue updating Xcode. The update process is stuck so when you press update nothing really happens.

The solution is:

1. delete Xcode (put it in the trash).
2. search for Xcode in the Mac App Store
3. do a fresh install of Xcode. (The button in the Xcode page in the Mac App Store will say "install" instead of "update" because you removed Xcode in step 2.)

Cheers!