Hello Lahmer Belkacem
Thank you for the data. I had a look at the DEM TIF you provided.
The issue is, that it uses a geographical coordinate system for georeferencing.
Unfortunately CityEngine only supports Cartesian coordinate systems for terrain layers.
Please export the DEM again using "WGS 1984 UTM Zone 31N" coordinates (same as OSM file) for georeferencing.
City Engine Help: Importing Georeferenced Data
"No reprojection nor rotation is applied to image data on import. The image's coordinate system is used only as reference system for location, extent and unit (translate and scale transformation) for the image data during import. It is therefore important to have image data ready in the target coordinate system before importing into CityEngine."
Thomas F