Terrain doesn't have a texture and can't edit it's height
Terrain must look as an untextured object, but instead it looks like that:
this is the default texture for a terrain.
click on the terrain and a menu should open up at the inspector, where you have a few options.
in the Project window, go to the assets folder and right click + pick create a new terrain layer.
than, you can drag this layer to the terrain and have a texture on it.
you can even drag more than one layers to have multiple textures on it.
on the height problem: try installing the unity terrain tools package or checking if the terrain is read only.
