First you need to clean up the perimeter of the terrain group. You need to open it for editing (right click on it and choose Edit group) and erase the bad geometry along the edges. Here I show some of it selected in yellow. Those faces are vertical and will prevent the object from being considered solid and 3D printable. I also softened the edges on the surface of the terrain to make it easier to see the stuff that needs fixing.
In order to make the base a “solid” and 3D printable, you need to add the geometry into the terrain group so that also requires having the group opened for editing.
Here I’m using a simpler example because your model is so huge it’s killing my graphics card. Draw vertical lines down from the corners and connect them with horizontal lines.
When you get finished, you should have faces on the sides and bottom and Entity Info should show the group as Solid.
Note that in order for the group to be considered a solid every edge in the group mush be shared by exactly two faces. No more and no less. This means no stray edges (edges not connected to any faces), no holes in the surface (edges only connected to one face) and no internal faces (edges shared by more than two faces.) Those vertical faces on the edges of your terrain will result in edges with only one face and that will be a problem.
If the perimeter of the terrain isn’t straight you will have to draw additional vertical lines down to be able to create the required faces.
Your profile says you are using the free web version of SketchUp. If that’s the case, you have a lot of manual work to do. If you were using SketchUp Pro there would be some extensions that could help you with this.