Hi Sara,
there are 2 main issues that usually explain this problem:
1. the raw topographical data have not been correctly imported, because their coordinates have been abnormally converted: in that case, you usually don't see the light blue points on the map after importing the XYZ file. If that's the case, go to Background Management and remove the background data, and then import the file again: while selecting the file to be imported, make sure to specifiy the appropriate coordinate system in "Convert from:"
2. if you can see the light blue points in the background, the problem may be that you haven't selected them for use in the interpolation. To select them, you have to click once on the "Import from background" icon, then select the points (e.g. drag a rectangle), and then click again on the icon to finalise the selection. Then you can interpolate and export again the dfs2 file.
Best regards,
Mathieu