Surface border issue with sandbox tools

Am trying to make sandbox grid solid or rather I want the terrain to be solid but when I check with solid inspector telling me surface boarder issue you need to fix manually

If you share your model we can see what the problem is…

Probably the surface borders are the edges of the sandbox surface. You need a skirt around the outside and a bottom.

In order for the object to be considered solid every edge must be shared by exactly two faces. No fewer and no mt.

Enough terrain volume what’s that? an extension or something

Yes, it’s an extension. But it is called Eneroth terrain volume.
But it’s easy enough to do it manually.
It’s better to learn how to do it yourself than relying on an extension…