I want to make the ground in this place of the terrain a bit deeper a bit lower but left shift key or right shift key and the left mouse button does not change anything on the terrain.
I've been looking for an answer for hours, and I finally found it. If you are trying to lower a piece of land that is already at 0 height, you will not be able. You can only lower areas that are above to height 0. In short, height 0 is the minimum: you cannot go to snow values due to the way the terrain is created. To be able to make valleys then you have to select the tool "Set Height", enter a height and raise all the ground to that height with "Flatten All", or small pieces of land by dragging the mouse on the ground. If you click "Flatten All", all the terrain will be brought to that height, losing all the changes made previously for the mountains. However, if you have already made land and you don't want to lose it and redo it from scratch, you can use the following solution:
You should now be able to create valleys! I haven't tried the process, but I've thought about it so much I'm pretty sure it will work.
Good work!