I\'m trying to build an extension project and Xcode keeps throwing the error in subject.

It might have happened that you duplicated your targets to do a clone app. For instance you selected each of your targets and duplicated them. When you cloned your watchkit extension target its target membership could be automatically assigned to the original watch app.
To fix this, you would need to go to yours original "WatchKit extension.appex" file in Project Navigator and unselect Target Membership in File Inspector for your new clone watch app target. You should do the opposite for your duplicated "Watchkit extension-copy.appex" and make sure it has only duplicated "watch app-copy" target selected there.