To understand why "key file" issues are so common, you need to look at the game’s troubled DRM history.
If the key says "Already in Use," you are likely logged into a different Ubisoft account than the one originally used to register the game. Anno 2070 Serial Number Key File
If you are looking for your own key today, the best place to check is your Ubisoft Connect account information or the "CD Key" section of the game's properties in your Steam library . To understand why "key file" issues are so