It looks like a problem with your installation of Extension Manager. Dreamweaver CC should be listed in the left panel of the extension manager. The error means that it thinks Dreamweaver isn't installed on your computer. Maybe you have a different version of extension manager that is newer and should be used for the installation. Find the one that lists the "Deprecated Server Behaviors" and install into the same area as that one since it would appear that version of Extension Manager is the currently active one.