I would help if you didn’t need to edit your question, rather that you had been more specific to begin with.
You need the cut cylinder to be a solid, so when you intersect it with the terrain you want the cylinder to cut the terrain too, therefore creating a cap for you cylinder.
Several ways to do it. Basically you want the cylinder and terrain in the same context when you intersect them. Whether that means using the raw geometry of both and making a hole in your terrain. Or using a copy of a section of the terrain pasted inside the cylinders group so that it’s non destructive to the original, depends on what you need.
It looks like the intersection is in the background on the terrain. If you can get that piece out and put it in place on the cut and it is a perfect fit it should form a solid and then you can read the volume in the Entity Info window.