I am afraid that I cannot even begin to understand whether this is a problem, let alone fix it if it is, if you do not directly answer the specific questions that I ask to try to understand the position.
In order to understand this, I need to see (1) a screenshot of your map generation and climate settings; and (2) a map generated from those exact settings. You have provided a screenshot and a map, but I have no idea whether they are in any way related to one another, and thus cannot make any use out of either of them.
In any event, looking at the map that you uploaded above, it is not remotely hilly: it is indeed really rather flat. Also, you have evidently loaded the map from a heightmap rather than generating it randomly, which means that the gradation of the map will be based on the heightmap, not the settings.
If you really want to flatten the heightmap, you could try manipulating the image in the GIMP (e.g. reducing the contrast), but I have no idea whether that would work (and cannot provide ongoing support for the details of how to do it or what to do if it does not work to your satisfaction).