Problem with GDS export of a layout in Agilent Advanced System Design (ADS)
The layer map is a file that has information on how to convert layout information from one format to another.

In your case (you're probably trying to export a layout to GDS) you need the layer map file. This file comes with the PDK of the technology that you are using. Refer to the PDK's documentation for where to find it, or contact whoever created the PDK and ask them to provide one.