After the completion of installing the adapter and there associated MODBUS Plus cabling. Can not access the controller, and code 818 is showing up.

 

I suggest that you double check to make sure you don't have any short circuits in your cabling, typically the middle ground wire will be making contact somewhere to one of the legs.

 

Do you have the proper end of line connectors?
You also have to have a minimum amount of resistance in your network. No shorts.
And you gotta be plugged into the MB+ socket.