Run the Avid Unity Client software installer. This will happen if the Avid Unity Client installer is run first and then Pro Tools is installed. This can also happen if Pro Tools software is upgraded. Specifically this has to do with the "UnityAsyncStreamsLib" file that should be located in the DAE folder of the Pro Tools system. This is file is placed by the Avid Unity Client software installer but I have seen cases where this file existed and Pro Tools was still unable to see Unity workspaces as valid record/playback drives. Running the Avid Unity Client software installer resolved it.