My FaceTime cannot be activated I got an iPad 2 running iOS 6.0.1. and I really have no idea why my FaceTime cannot be activated.

I always got an error while log in (username or password is incorrect)

But actually the Apple ID and password is correct and it can be used for downloading apps and iMessage.
This error is only set to appear if your username or password is indeed incorrect, it doesn't fire by random. My advice would be to reset your password using iForgot.