Dear NIrdesh Singh,
CityEngine only allows projected coordinate systems (PCS) as scene coordinate systems.
Coordinate System Dialogs
For importing base maps, it is important to know, that CityEngine does not support re-projection.
Therefore the imported raster data needs to be in the same coordinate system as your scene.
Importing Georeferenced Data
Best practice for CityEngine is to use a national grid or UTM as scene coordinate system.